Fix dead link, improve Mac OSX recognition

The first change just fixes a dead link.  The second change tries to improve the recognition of the MacOSX platform as sometimes it is still returned as a unix platform even if on mac (see: http://mono.wikia.com/wiki/Detecting_the_execution_platform).

I am not actually sure changing the "DYLD_FALLBACK_LIBRARY_PATH" variable has any effect at this stage though since the program is already loaded.