WITH_SYSTEM_COMPILER: Avoid using GCC4.8+ logic on native GCC4.2 platforms.

As the XXX notes, these should really be checking MK_GNUCXX since there is
already a version check in share/mk/src.opts.mk to disable it. Fixing that
here is more complex though. This could also be using X_COMPILER_FEATURES
but uses X_COMPILER_VERSION to keep in sync with the src.opts.mk logic.

Tested by: andreast
Sponsored by: EMC / Isilon Storage Division
Approved by: re (gjb)