Issue #15968: Incorporated Tcl, Tk, and Tix builds into the Windows build
solution.

Currently, Tix is not built in Debug configuration.

This change also:
- simplifies some Tcl/Tk-related msbuild properties for _tkinter
- copies the Tcl and Tk DLLs into the build output directory, meaning
  they will always be available after a build without having to copy
  them manually or change PATH
- removes PCbuild/build_tkinter.py: the solution does the build without
  needing to invoke Python (so Tcl/Tk/Tix can be built in parallel with
  the rest of the build using the `/m` msbuild command line switch)
- removes an outdated README concerning building Tcl/Tk on AMD64
diff --git a/PCbuild/_tkinter.vcxproj b/PCbuild/_tkinter.vcxproj
index 67f72fe..9218d63 100644
--- a/PCbuild/_tkinter.vcxproj
+++ b/PCbuild/_tkinter.vcxproj
@@ -85,41 +85,49 @@
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd.props" />
     <Import Project="pgupdate.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <ImportGroup Condition="'$(Configuration)|$(Platform)'=='PGInstrument|Win32'" Label="PropertySheets">
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd.props" />
     <Import Project="pginstrument.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'" Label="PropertySheets">
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="PropertySheets">
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd_d.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <ImportGroup Condition="'$(Configuration)|$(Platform)'=='PGUpdate|x64'" Label="PropertySheets">
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd.props" />
     <Import Project="x64.props" />
     <Import Project="pgupdate.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <ImportGroup Condition="'$(Configuration)|$(Platform)'=='PGInstrument|x64'" Label="PropertySheets">
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd.props" />
     <Import Project="x64.props" />
     <Import Project="pginstrument.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'" Label="PropertySheets">
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd.props" />
     <Import Project="x64.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'" Label="PropertySheets">
     <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
     <Import Project="pyd_d.props" />
     <Import Project="x64.props" />
+    <Import Project="tcltk.props" />
   </ImportGroup>
   <PropertyGroup Label="UserMacros" />
   <PropertyGroup>
@@ -155,7 +163,7 @@
       <PreprocessorDefinitions>WITH_APPINIT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
     <Link>
-      <AdditionalDependencies>$(tcltkLibDebug);%(AdditionalDependencies)</AdditionalDependencies>
+      <AdditionalDependencies>$(tcltkLib);%(AdditionalDependencies)</AdditionalDependencies>
     </Link>
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
@@ -163,11 +171,11 @@
       <TargetEnvironment>X64</TargetEnvironment>
     </Midl>
     <ClCompile>
-      <AdditionalIncludeDirectories>$(tcltk64Dir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+      <AdditionalIncludeDirectories>$(tcltkDir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
       <PreprocessorDefinitions>WITH_APPINIT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
     <Link>
-      <AdditionalDependencies>$(tcltk64LibDebug);%(AdditionalDependencies)</AdditionalDependencies>
+      <AdditionalDependencies>$(tcltkLib);%(AdditionalDependencies)</AdditionalDependencies>
     </Link>
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
@@ -184,11 +192,11 @@
       <TargetEnvironment>X64</TargetEnvironment>
     </Midl>
     <ClCompile>
-      <AdditionalIncludeDirectories>$(tcltk64Dir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+      <AdditionalIncludeDirectories>$(tcltkDir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
       <PreprocessorDefinitions>WITH_APPINIT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
     <Link>
-      <AdditionalDependencies>$(tcltk64Lib);%(AdditionalDependencies)</AdditionalDependencies>
+      <AdditionalDependencies>$(tcltkLib);%(AdditionalDependencies)</AdditionalDependencies>
     </Link>
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='PGInstrument|Win32'">
@@ -205,11 +213,11 @@
       <TargetEnvironment>X64</TargetEnvironment>
     </Midl>
     <ClCompile>
-      <AdditionalIncludeDirectories>$(tcltk64Dir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+      <AdditionalIncludeDirectories>$(tcltkDir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
       <PreprocessorDefinitions>WITH_APPINIT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
     <Link>
-      <AdditionalDependencies>$(tcltk64Lib);%(AdditionalDependencies)</AdditionalDependencies>
+      <AdditionalDependencies>$(tcltkLib);%(AdditionalDependencies)</AdditionalDependencies>
       <TargetMachine>MachineX64</TargetMachine>
     </Link>
   </ItemDefinitionGroup>
@@ -227,11 +235,11 @@
       <TargetEnvironment>X64</TargetEnvironment>
     </Midl>
     <ClCompile>
-      <AdditionalIncludeDirectories>$(tcltk64Dir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+      <AdditionalIncludeDirectories>$(tcltkDir)\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
       <PreprocessorDefinitions>WITH_APPINIT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
     <Link>
-      <AdditionalDependencies>$(tcltk64Lib);%(AdditionalDependencies)</AdditionalDependencies>
+      <AdditionalDependencies>$(tcltkLib);%(AdditionalDependencies)</AdditionalDependencies>
       <TargetMachine>MachineX64</TargetMachine>
     </Link>
   </ItemDefinitionGroup>
@@ -244,6 +252,12 @@
       <Project>{cf7ac3d1-e2df-41d2-bea6-1e2556cdea26}</Project>
       <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
     </ProjectReference>
+    <ProjectReference Include="tcl.vcxproj">
+      <Project>{b5fd6f1d-129e-4bff-9340-03606fac7283}</Project>
+    </ProjectReference>
+    <ProjectReference Include="tk.vcxproj">
+      <Project>{7e85eccf-a72c-4da4-9e52-884508e80ba1}</Project>
+    </ProjectReference>
   </ItemGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">