mk: fix build with Debian/Ubuntu-specific gcc version

mk: fix build with Debian/Ubuntu-specific gcc version

Commit 71f0ab1849b4fc3ca928deb566df12ca725ed150 broke compilation
on some versions of Debian and Ubuntu where gcc has been modified
to only emit MAJOR.MINOR part of the version from 'gcc -dumpversion'.
Drop the micro-version from gcc version comparisons to work around
this, it wasn't being used for anything anyway.

Fixes: 71f0ab1849b4 ("mk: rework gcc version detection to permit versions newer than 4.x")
Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Acked-by: David Marchand <david.marchand@6wind.com>