Unlike the compiler binary name search logic, the `setup_environment` in
GCC's package assumes the compiler names are *exactly* `gcc`, `g++`,
etc. In many external installations (Homebrew, Macports) the
installation includes only *suffixed* versions such as `gcc-9`.
This patch uses the GCC compiler search suffixes to actually locate the
correct filenames for the installed compilers, allowing the
Spack-generated module file to have useful definitions of CC, CXX, etc.
It also allows for the possibility that the user's external installation
of GCC is compiled without Fortran support, in which case the `FC`
environment variables are not defined.
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