extend/os/linux/system_config: support gcc 12 & keg-only glibc
extend/os/linux/system_config: support gcc 12 & keg-only glibc

MikeMcQuaid/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,nandub/brew,Homebrew/brew,JCount/brew,JCount/brew,Homebrew/brew,JCount/brew,Homebrew/brew,nandub/brew,Homebrew/brew,JCount/brew,nandub/brew,nandub/brew