FIX #294: fork-safe pthread mode

This is a fix for #294 (OpenBLAS hangs when calling DGEMM after a Unix fork) when OpenBLAS is built with `make USE_OPENMP=0`.

When built with OpenMP and GCC/libgomp, the freeze will still happen and can only be fixed in the libgomp library itself, for instance by:

  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60035

The CPU affinity (if any) is not reset upon forking. This will be addressed in a separate PR.