[patch][non-maintainer] add use_gmake to games/flightgear

Per bento, this port seems to require having USE_GMAKE set.
(This probably doesn't really affect most users). Also,
while fiddling around, pacify portlint a bit.

Informed maintainer.

PR: ports/56514
Submitted by: Mark Linimon <linimon@lonesome.com>