test build with Clang with and without libc++ on OSX

[torbjoernk]

LLVM's implementation of the STL, libc++ is officially supported only on Mac OS. We have some CMake logic to use libc++ when compiling with Clang but it needs explicit activation/deactivation on non-OSX when compiling with Clang.

Due to lack of a Mac machine, I can't test that logic and need some help here. We might need to add another check on whether CMake's variable APPLE is true or not.

[memmett]

@torbjoernk Looks as though you resolved this already -- if so, can you close this issue?

[torbjoernk]

I'd like to have a few more people testing that before closing the issue.