gcc@5: Fix glibc installation logic
gcc@5: Fix glibc installation logic

See https://github.com/Homebrew/brew/pull/13231#issuecomment-1115417732

This code was present at one point in linuxbrew-core but we made
an error while migrating it.
stgraber/homebrew-core,makigumo/homebrew-core,adam-moss/homebrew-core,lemzwerg/homebrew-core,Homebrew/homebrew-core,zyedidia/homebrew-core,nandub/homebrew-core,filcab/homebrew-core,nbari/homebrew-core,stgraber/homebrew-core,makigumo/homebrew-core,zyedidia/homebrew-core,filcab/homebrew-core,sjackman/homebrew-core,Homebrew/homebrew-core,nbari/homebrew-core,chrisfinazzo/homebrew-core,sjackman/homebrew-core,lemzwerg/homebrew-core,adam-moss/homebrew-core,chrisfinazzo/homebrew-core,nandub/homebrew-core