Test build with Homebrew gcc/gfortran instead of fxcoudert installer

[karlglazebrook]

The current SciPDL build expects gcc and gfortran from François-Xavier Coudert's standalone macOS installer at https://github.com/fxcoudert/gfortran-for-macOS/releases (installed under /usr/local/gfortran/ with symlinks at /usr/local/bin/gcc and /usr/local/bin/gfortran). The gfortran-static wrapper script hardcodes /usr/local/bin/gfortran.

Homebrew also offers brew install gcc which provides gcc-14, gfortran-14 etc. under /opt/homebrew/bin/. This has not been tested with the SciPDL build flow as of v2.104.

Tasks

• Try a clean build with Homebrew GCC instead of fxcoudert's GCC
• Likely needs gfortran-static updated to point at /opt/homebrew/bin/gfortran-14 (or whatever the current Homebrew naming is)
• Check that the various manual gfortran re-link commands in build_scipdl.sh (Slatec, Minuit, FFTW3, GD bundle, etc.) all still work correctly
• Verify the static linking behavior is the same (-static-libgfortran -static-libgcc -static-libquadmath flags work identically)
• If it works cleanly, switch the documented setup to Homebrew GCC (one less prerequisite step) and update BUILDING.md

Why bother

• One less manual installer step for new builders — brew install gcc is friendlier than downloading a .dmg from a personal GitHub release
• Easier to keep up to date alongside other Homebrew packages
• More consistent with the Homebrew-centric build environment we already require for libpng/libx11/etc.

See BUILDING.md for current setup.