NMake: Add warning messages about stub python.exe

Hi,

This attempts to address a possible issue raised by @wroy in !824 (merged) where if one does not specify PYTHON in the NMake command line for preparing the build from a GIT checkout, or when building introspection or when creating the pkg-config file upon "install", a stub python.exe (which won't work unless Python is installed from the Microsoft Store) will be run instead of the one that the user has in %PATH%.

In cases when PYTHON=xxx is not specified in the NMake command line and a stub python.exe is found in one's %LOCALAPPDATA%, display a message that advises one to specify PYTHON=<path_to_real_python_interpreter> if the build fails.

The stub python.exe was initially meant for Microsoft to point one to install a Microsoft Store version of Python, which sadly can get into the way for us.

With blessings, thank you!