Clang always prints "clang $VERSION" regardless of the name used to
execute it, whereas GCC prints "$progname $VERSION", meaning if CC is
set to cc and cc is GCC it will print "cc $VERSION". We are able to
detect some of those cases since it then prints "($PKGVERSION)", where
the default is "GCC", but many distributions override that to print
their name and the package version number (e.g. "Debian 10.2.1-6"), so
nothing tells us it's GCC other than the fact that it's not Clang (and
that there's an FSF copyright disclaimer).
However, GCC's -v option will always print "gcc version $VERSION", so
fall back on using that to detect GCC. Whilst Clang also supports this
option, we should never get here, so Clang handling is not added.