[API/FIX] add FreeBSD support to build system

Tested on FreeBSD 10.0. Both gcc49 and system compiler (clang with libc++) tested.
[BUILD] reverse 920d338ac1ef9d294a34249de33b02fafeed57e7 partially
[BUILD] moved platform to top and exit with warning if unknown platform is detected
[INTERNAL] add SEQAN_FORWARD_RETURN; should be good from my POV