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.

doc/fixes3-7-0.txt

@@ -2863,9 +2863,8 @@ use %lu, not %d, in format string in timer_sanity_check() (pr #617 by argrath)
 bad cast making sp_lev chameleon light source (pr #625 by entrez)
 add Ray Chason's adaptation of nethack's Qt5 interface to work with Qt6 (issue
 	#525 followup comment by chasonr)
-mingw32 build updates and replacement of sys/windows/Makefile.gcc with new
-	sys/windows/Makefile.mingw32 and sys/windows/Makefile.mingw32.depend
-	(pr #661 by feiyunw)
+mingw32 build updates to replace contents of sys/windows/GNUmakefile and new
+	sys/windows/GNUmakefile.depend (pr #661 by feiyunw)
 mark various pointers to const char as const pointers (pr #624 by argrath)
 function fill_special_room() in sp_lev.c was dereferencing a pointer
 	argument prior to a subsequent check for a NULL pointer that

doc/sound.txt

@@ -508,8 +508,8 @@ use the following guidelines:
                         Windows with Visual studio nmake at the command
                         line.
 
-       sys/windows/Makefile.mingw32
-       sys/windows/Makefile.mingw32.depend
+       sys/windows/GNUmakefile
+       sys/windows/GNUmakefile.depend
 
                         Will require updates in order to build on
                         Windows with mingw32 or MSYS2 using GNU make at