Enable FreeBSD build, fix #378

[illwieckz]

Tested on FreeBSD 11.1 and GCC 5 (default) and GCC 6 (and with GCC 8 with fix from #564).
Compilation went well for all the binaries provided by the repository.

q3map2 runs but I was not able to test radiant: I get an early segfault but it means nothing at this point: even the default provided gdb mismatches with the installed libstdc++ ¯\_(ツ)_/¯, so I'm even not able to debug: the distro seems borked.

Edit: it works once scons uses the system compiler (based on clang) instead of gcc.

It fixes #378.

[illwieckz]

I told scons to use the default FreeBSD's compiler (which is clang 4.0) and it build and runs. I got a GL crash but that's probably because I was running through an X11 Forwarding through an SSL tunnel without any direct rendering. 😁

Note: if a compilation was attempted with gcc first, you must delete site.sconf and clean the source tree doing scons -c before building again.

[illwieckz]

It works: