Ignore OpenMP on machine that does not have it

Compilers are supposed to ignore #pragma directives they don't understand; that's the whole point of the syntax. And the functions defined in openmp.h have simple well-defined meanings on a non-parallel system -- in particular, the header file will check for whether the compiler defines ENABLE_OPENMP and, if it's not enabled, provide the right fallbacks.

So, all you need is a copy of openmp.h to link to. Here's one: http://cms.mcc.uiuc.edu/qmcdev/docs/html/OpenMP_8h-source.html .

The relevant portion of the code, though, is just this:

#if defined(ENABLE_OPENMP)
#include <omp.h>
#else
typedef int omp_int_t;
inline omp_int_t omp_get_thread_num() { return 0;}
inline omp_int_t omp_get_max_threads() { return 1;}
#endif

At worst, you can just take those three lines and put them in a dummy openmp.h file, and use that. The rest will just work.

OpenMP compilation adds the preprocessor definition "_OPENMP", so you can do:

#if defined(_OPENMP)
   #pragma omp ...
#endif

For some examples, see http://bisqwit.iki.fi/story/howto/openmp/#Discussion and the code which follows.