Add -std=legacy to FFLAGS.
Do the same for the Windows package, even though we don’t yet use version 10 of
gfortran (but that will of course happen at some point).
* The macOS snapshot until this point did not work unless the user had already installed `libmatio`, `gsl`, and `gcc` via Homebrew.
* We are not modifying `m4/ax_matio.m4` as we want to still be able to compile against shared libraries in development
Sébastien Villemot
Houtan Bastani