allow PRPACK to be compiled on clang (which does not support OpenMP yet)

This patch updates `makefile` and `prpack_driver_benchmark.cpp` to enable compilation of PRPACK using `clang`, which does not support OpenMP extensions yet. The primary reason for this patch is that Mac OS X 10.9 ships `clang` as the default compiler (no `gcc`).

OpenMP support can be disabled by compiling PRPACK with `OPENMP_SUPPORT=0 make`; this will remove the `-fopenmp` flag from `CXXFLAGS` and add `-Wno-unknown-pragmas` instead to ask `clang` not to complain about the OpenMP pragmas.

Also, `omp.h` is now included conditionally in `prpack_driver_benchmark.cpp`. In the absence of OpenMP support, the file now provides fake implementations of `omp_get_max_threads` and `omp_set_num_threads`.