GHA: Add MSYS2 build

[zmughal]

This PR is on top of the commits of
#2 and
#3.

This PR also requires

• disabling some of the routines in sys/ and some of the drivers in
  drivers/ because of required headers that are not available under MinGW:
  • require termios.h: sys/grgetc.c, sys/grtermio.c,
    drivers/ttdriv.f, drivers/vtdriv.f
  • require sys/socket.h: drivers/nexsup.c, drivers/nedriv.f
• setting FC to gfortran because f77 is not available (on Linux f77 is
  a symlink to gfortran)
  • FC = f77 means fonts/pgpack can not build
  • this can be worked around by setting FC to the contents of F77 (gfortran)

[djerius]

I've created a new branch based off of this one which conditionally compiles the stubbed out drivers if the system provides the capabilities. Seems to have worked

[zmughal]

OK, I'm going to merge and squash that commit into this PR.

[djerius]

The build error seems to be due to autoconf setting EXEEXT = .exe which is causing make to use a built-in recipe for compiling Fortran programs without linking.

On MSYS (and on Linux if I copy over the Makefile) with the above setting for EXEEXT,

make -n
f77 -g -O2   pgpack.f   -o pgpack
rm -f grfont.dat
./pgpack < ./grfont.txt
make  all-am

If make was using the build recipes in the Makefile, it would use the F77COMPILE make macro, which resolves to gfortran -std=legacy -g -O2

Setting EXEEXT = gives the expected result.

make -n
gfortran -std=legacy -g -O2 -c -o pgpack.o pgpack.f
rm -f pgpack
/bin/sh ../libtool  --tag=F77   --mode=link gfortran -std=legacy -g -O2   -o pgpack pgpack.o  -lm  -LD:/a/_temp/msys64/mingw64/lib -lpng16 -lz 

From the autoconf manual

All the tests for compilers ('AC_PROG_CC', 'AC_PROG_CXX', 'AC_PROG_F77')
define the output variable 'EXEEXT' based on the output of the compiler,
typically to the empty string if Posix and '.exe' if a DOS variant.

I don't know much about MSYS, but I'm guessing that it doesn't need the .exe to indicate that it's an executable, but autoconf doesn't know that.

[djerius]

I don't know much about MSYS, but I'm guessing that it doesn't need the .exe to indicate that it's an executable, but autoconf doesn't know that.

Or, it could be due to one of the mst_* autoconf macros that I'm responsible for.

[djerius]

I see you found the bug as well. Please rebase against main, as I fixed an autoconf macro.

[zmughal]

Found it thanks to your debugging in the comments above! Rebasing now. :-)

[zmughal]

CI passes. Should be ready to review now.