FreeBSD/ports 484471head/devel/simgear Makefile, head/games/flightgear Makefile

devel/simgear: GCC architectures require linking with libosgDB [1]

Also, remove support for Clang < 4.0 and GCC < 6 for devel/simgear [1] and
games/flightgear ports.

PR:            232832 [1]
Submitted by:   Piotr Kubaj <pkubaj at anongoth.pl> [1]