macOS: bump to GCC 11 (for compilation and for use_dll)

The formulas are taken from commit 2a3ba743401693c5927acc936e27377f123f6d56 of
the homebrew-core repository.

The gcc.rb formula was modified by adding “cellar: :any” to all bottles (see
https://github.com/Homebrew/brew/blob/master/docs/Bottles.md).

The preprocessor has been updated accordingly (and the detection of the GCC
installed by Dynare package has also been fixed).