On Linux, if you can't actually locate the libc++ install, do not attempt to use it

The failure mode when one gets this wrong is quite gnarly to then walk oneself out of, and if you can't actually find the library, trying to build against it is fairly pointless anyway
This + my previous skip_if_library_missing change should make running the libc++ tests on a Linux machine without it much more seamless

llvm-svn: 248057