Fix x13 on Mac

A user at https://forum.dynare.org/t/x13-seasonal-adjustedment-does-not-work-on-mac/20898 reports an issue with compiling x13, which seems to be due to a conflicting GCC versions. The fix to

Library not loaded: '/usr/local/opt/gcc/lib/gcc/11/libgfortran.5.dylib'

is most likely to statically link libgfortran