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.

src/.gitignore

@@ -28,6 +28,6 @@ objutil/*
 objlua/*
 objpdc/*
 bundle/*
-Makefile.mingw32
-Makefile.mingw32.depend
+GNUmakefile
+GNUmakefile.depend
 .*.c