Submitting this as a PR to share what is needed to make

spack:
  specs:
  - "fftw~mpi %c,fortran=llvm"
  packages:
    c:
      prefer: [gcc]
    cxx:
      prefer: [gcc]
    fortran:
      prefer: [gcc]

pick gmake %c=gcc instead of gmake %c=llvm.

The changes in this PR are the minimum needed to make "preferred compilers" the tie-breaker.

But these change make existing tests fail (e.g. foo %clang is expected to set %clang on deps too), so, that suggests that the penalty from requirements (on providers) should be in terms of count of parent packages using them instead of count of providers existing.

Edit: Closed this as @alalazo has an implementation of the suggestion in the PR description.