https://bugzilla.redhat.com/show_bug.cgi?id=994589 complained that
even when using a cross-compiler not named 'gcc', the configure
output confusingly referred to gcc.
* m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Use a
more generic statement in configure output.
Signed-off-by: Eric Blake <eblake@redhat.com>