Improvements to Visual Studio builds

Hi,

This attempts to improve Visual Studio builds by:

• Fixing builds especially when introspection builds are not done and when python.exe is in the PATH. This moves where we set up the default value for $(PYTYHON) in the NMake Makefiles so that it can be consumed for all builds, and also add a similar setup for PERL, so that they can be called by the NMake Makefiles directly when they are in the PATH, or can be in-turn called when their full paths are passed into the NMake command line via PYTHON=... and PERL=....

• Add support for building ARM64 Windows binaries in the NMake Makefiles.

With blessings, thank you!