NMake Makefiles: Improve docs on building from GIT checkouts

Hi,

As pointed out in #970, we can make things clearer to people when building from a GIT checkout:

• Mention that nmake /f generate-msvc.mak generate-nmake-files ... is necessary prior to running the actual build if building from a GIT checkout.
• Onlyshow clearer diagnostic message when running nmake /f generate-msvc.mak generate-nmake-files without PYTHON=... specified.

With blessings, thank you!