Limit catopen usage to unix-like OSes

Operating systems that are not unix-like are unlikely to have access to
catopen. Instead of black-listing each one, we now filter out all non-unix
operating systems first. We then exclude the unix-like operating systems
that don't have catopen. _WIN32 counts as a unix-like operating system
because of cygwin.

http://reviews.llvm.org/D16639

llvm-svn: 260381