games/megaglest: Unbreak on F10+ with an unsavory solution

Megaglest cannot be built under c++11 standard from the latest clang.
It catches a narrowing error. The real solution is to fix the c++
code (it wasn't the only problem, but the first to trigger an error over
a warning). The c++11 standard is required for wx, so it can't be
unspecified. It turns out that gcc48 will not catch the error that clang
does so the temporary fix is to require gcc48 for all FreeBSD releases.
This may work for gcc49 as well, but it's likely that gcc5 will also
catch the bug and break.

So the unsavory short-term solution is to require GCC to build megaglest,
but the long term fix is either patch the code or hopefully update to
new version when upstream fixes the bug (then USE_GCC can be removed).

Reported by: amdmi3