provide better support (docs?) for building with clang

[dimpase]

While INSTALL has NOTE TO MAC OS users, which basically says "use real gcc" (without explicitly saying "use real gcc, not apple's clang pretending to be gcc"), people mixing clang and gfortran are potentially in trouble. One ends up with having both libomp and libgomp linked to libmsolve, which isn't what one wants.

clang doesn't work with libgomp, it needs libomp, and gfortran can work with libomp, so one needs to be careful with passing -fopenmp to gfortran in linking mode (?).

Here is a reproducer, on Linux:

CC=clang ./configure --prefix=/tmp/
CC=clang make
ldd /tmp/lib/libmsolve.so
	linux-vdso.so.1 (0x00007f27771cf000)
	libneogb.so.0 => /usr/local/lib/libneogb.so.0 (0x00007f2777141000)
	libflint.so.19 => /usr/lib64/libflint.so.19 (0x00007f2776600000)
	libmpfr.so.6 => /usr/lib64/libmpfr.so.6 (0x00007f277654b000)
	libgmp.so.10 => /usr/lib64/libgmp.so.10 (0x00007f27764a7000)
	libm.so.6 => /usr/lib64/libm.so.6 (0x00007f27763f1000)
	libomp.so => /usr/lib64/libomp.so (0x00007f27762c0000)
	libc.so.6 => /usr/lib64/libc.so.6 (0x00007f27760eb000)
	libgomp.so.1 => /usr/lib/gcc/x86_64-pc-linux-gnu/14/libgomp.so.1 (0x00007f2776095000)
	libcblas.so.3 => /usr/lib64/libcblas.so.3 (0x00007f277605b000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f27771d1000)
	libblas.so.3 => /usr/lib64/libblas.so.3 (0x00007f2775fb4000)
	libgfortran.so.5 => /usr/lib/gcc/x86_64-pc-linux-gnu/14/libgfortran.so.5 (0x00007f2775c00000)
	libgcc_s.so.1 => /usr/lib/gcc/x86_64-pc-linux-gnu/14/libgcc_s.so.1 (0x00007f2775f86000)

Here is a related story for OpenBLAS: OpenMathLib/OpenBLAS#5169
and OpenMathLib/OpenBLAS#5180

[wegank]

In the meantime, we should really drop the suggestion to use real GCC on macOS, as it's simply not the case there: Homebrew (via Macaulay2's tap) builds msolve with Apple Clang, and Nix builds msolve with LLVM Clang...

[dimpase]

Homebrew now has msolve in its core, not in a tap.
The build script (Formula) explicitly sets flags for LLVM's OpenMP library