NMake Makefiles: More backports of items from the main branch

Chun-wei Fan requested to merge windows-store-python-2.56 into librsvg-2.56


This MR covers for the librsvg-2.56 branch, for:

  • Parts of !824 (merged) for a default value for PYTHON, as well as adding $(INCLUDEDIR)\cairo in the locations to look for the Cairo headers.
  • Backport of !847 (merged) regarding the possible use of a stub python.exe during the build.

With blessings, thank you!

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