devel/libvirt: depend on GNU grep(1) in a more robust way

FreeBSD ships native grep(1) implementation in the base for
quite a while, so don't bother to check its origin and just
request GNU version explicitly like on Darwin (macOS).

Depending on `/usr/local/bin/grep' is also wrong because of
the hardcoded prefix and because compatibility symlinks are
optional (plans are to disable them in the future).