GFortran chokes when it is passed CPU flags specific to Clang. This change ensures the environment variables `FCFLAGS` and `FFLAGS` contain the same CPU flags that would be set for the GCC compiler. Fixes Homebrew/homebrew#10424. Fixes Homebrew/homebrew#10744. Closes Homebrew/homebrew#10774. Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>