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@anongoth.pl> [1]