Try to fix Windows builds for CPython some more

Author: saudet

cpython/cppbuild.sh

@@ -173,7 +173,8 @@ case $PLATFORM in
     windows-x86)
         mkdir -p ../include ../lib ../libs ../bin
         cd PCbuild
-        PLATFORM= cmd.exe //c 'build.bat -p x86 -vv'
+        unset PLATFORM
+        cmd.exe //c 'build.bat -p x86 -vv'
         cp win32/python*.exe win32/python*.dll ../../bin/
         cp win32/python*.lib ../../libs/
         cp win32/*.dll win32/*.pyd ../../lib/
@@ -186,7 +187,8 @@ case $PLATFORM in
     windows-x86_64)
         mkdir -p ../include ../lib ../libs ../bin
         cd PCbuild
-        PLATFORM= cmd.exe //c 'build.bat -p x64 -vv'
+        unset PLATFORM
+        cmd.exe //c 'build.bat -p x64 -vv'
         cp amd64/python*.exe amd64/python*.dll ../../bin/
         cp amd64/python*.lib ../../libs/
         cp amd64/*.dll amd64/*.pyd ../../lib/