guard use of feenableexcept, which does not exist on OS X

On OS X, feenableexcept does not exist, so scuff-em does not compile without this change. I only deactivated the possibility to abort on floating point exceptions, it should in principle also be possible to enable it using a different approach, see [this stackoverflow question](http://stackoverflow.com/questions/247053/enabling-floating-point-interrupts-on-mac-os-x-intel).