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- removes an outdated README concerning building Tcl/Tk on AMD64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/debug.props b/PCbuild/debug.props
index 9b7a65a..54f3c32 100644
--- a/PCbuild/debug.props
+++ b/PCbuild/debug.props
@@ -3,6 +3,7 @@
   <PropertyGroup Label="UserMacros">
     <PyDebugExt>_d</PyDebugExt>
     <KillPythonExe>$(OutDir)kill_python_d.exe</KillPythonExe>
+    <TclDebugExt>g</TclDebugExt>
   </PropertyGroup>
   <PropertyGroup>
     <_ProjectFileVersion>10.0.30319.1</_ProjectFileVersion>
@@ -23,5 +24,8 @@
     <BuildMacro Include="KillPythonExe">
       <Value>$(KillPythonExe)</Value>
     </BuildMacro>
+    <BuildMacro Include="TclDebugExt">
+      <Value>$(TclDebugExt)</Value>
+    </BuildMacro>
   </ItemGroup>
 </Project>
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