rename sys/windows/Makefile.mingw32 to GNUmakefile

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.

sys/windows/nhsetup.bat

@@ -47,10 +47,10 @@ echo           ..\..\src\Makefile-orig
 copy /Y Makefile.nmake ..\..\src\Makefile >nul
 echo Microsoft nmake Makefile.nmake copy to ..\..\src\Makefile completed.
 
-echo Copying mingw-w64 Makefile.mingw32 to ..\..\src\Makefile.mingw32
-copy /Y Makefile.mingw32 ..\..\src\Makefile.mingw32 >nul
-echo Copying mingw-w64 Makefile.mingw32.depend to ..\..\src\Makefile.mingw32.depend
-copy /Y Makefile.mingw32.depend ..\..\src\Makefile.mingw32.depend >nul
+echo Copying mingw-w64 GNUmakefile to ..\..\src\GNUmakefile
+copy /Y GNUmakefile ..\..\src\GNUmakefile >nul
+echo Copying mingw-w64 GNUmakefile.depend to ..\..\src\GNUmakefile.depend
+copy /Y GNUmakefile.depend ..\..\src\GNUmakefile.depend >nul
 echo mingw-w64 Makefile copies to ..\..\src completed.
 
 echo Done copying files.