NMake Makefiles: Fix introspection for release tarballs

Hi,

When I added items to allow building from GIT checkouts for Visual Studio, I had the NMake Makefiles check for items that only exist for GIT checkouts for various items that we use for the build, which broke introspection for Visual Studio builds from release tarballs. This attempts to fix things by only checking for the items when we are indeed building from a GIT checkout, and improve things by silencing out the commands that we use to generate the file listings for building the introspection files.

With blessings, thank you!