Add cmake check for which C++ stdlib clang is using

This is neccessary b/c Mac OS X started linking clang to libc++
Fix checking for any OpenCL library
Clear up messages on static linking
One more C++ stdlib fix
Fix statically linking LLVM into libpocl.so

- Order of static clang/LLVM matters, libclang* must go first
- POCL_LLVM_LIBS is a string, turn it into a list