The mainline version of clang doesn't support openmp yet, but support seems to be well on its way (http://clang-omp.github.io/).

Here is my rationale replacing GXX with CXX:

On OSX 10.9, g++ is an alias for clang++, so using GXX=g++ is an unreliable way of setting the compiler. Besides, the Makefile already includes a separate check for clang to disable openmp at compile time (actually, we may want to emit a $(warning) in this case).

Most importantly, however, I think we really should honor the user's choice of CXX when compiling the examples. Otherwise we can end up building libHalide with a different compiler than the examples, which will usually result in linking errors: clang-3.2, clang-3.4 and g++ are all mutually incompatible due to differences in their standard libraries. This actually bit me when I used CXX=clang++-3.4.