Fixes Tix build by correcting the directories used by Tcl and Tk. (#3391)

diff --git a/PCbuild/tk.vcxproj b/PCbuild/tk.vcxproj
index a1b8b9e..70b5459 100644
--- a/PCbuild/tk.vcxproj
+++ b/PCbuild/tk.vcxproj
@@ -54,7 +54,7 @@
   <PropertyGroup>
     <TkOpts>msvcrt</TkOpts>
     <TkOpts Condition="$(Configuration) == 'Debug'">symbols,msvcrt</TkOpts>
-    <TkDirs>TCLDIR="$(tclDir.TrimEnd(`\`))" INSTALLDIR="$(OutDir.TrimEnd(`\`))"</TkDirs>
+    <TkDirs>BUILDDIRTOP="$(BuildDirTop)" TCLDIR="$(tclDir.TrimEnd(`\`))" INSTALLDIR="$(OutDir.TrimEnd(`\`))"</TkDirs>
     <DebugFlags Condition="'$(Configuration)' == 'Debug'">DEBUGFLAGS="-wd4456 -wd4457 -wd4458 -wd4459 -wd4996"</DebugFlags>
     <WarningsFlags>WARNINGS="-W3 -wd4244 -wd4267 -wd4311 -wd4312 -wd4334"</WarningsFlags>
     <NMakeBuildCommandLine>setlocal