GNU make looks first for a file called GNUmakefile, ahead of
looking for Makefile and then makefile.
Renaming sys/windows/Makefile.mingw32 to sys/windows/GNUmakefile
allows:
o src/GNUmakefile (for use by GNU make) and src/Makefile (for use
Microsoft nmake) to both reside in the src folder during build.
o src/GNUmakefile will be used by GNU make, without having to
explicitly specify "-f GNUmakefile" on the GNU make command line.
o src/Makefile will be used by Microsoft nmake, without having to
explicitly specify "-f Makefile" on the Microsoft nmake command line.
For the gcc build, the movemement of sys/windows/GNUmakefile needs
to be copied to src/GNUmakefile as part of the build process (see
sys/windows/build-msys2.txt).
For the Microsoft Visual Studio command line build with nmake,
sys/windows/Makefile.nmake needs to be copied to src/Makefile as
part of the build process (see sys/windows/build-nmake.txt).
They are both copied to the src folder from their respective
repository source file names when the nhsetup.bat file is used.
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