Added __BSD_VISIBLE 1 to allow compiling on FreeBSD.

SIGWINCH is defined in signal.h, but on FreeBSD SIGWINCH is guarded by __BSD_VISIBLE as it's not really POSIX.  I've added __BSD_VISIBLE to the main nyan file to let this compile.

I tested compilation on OSX Mavericks and 12.04 ubuntu still functions, and I compiled my code on FreeBSD 10.0.

I have not tested installation under FreeBSD.