Enable OpenMP support for john --jumbo

This enables OpenMP support in john, if it is compiled with the jumbo
patch and openmp support is available. It is only enabled with the jumbo
patch, because regular john makes no use of openmp even if enabled.

Clang does not currently support OpenMP, so when building with clang OpenMP is disabled.
Looking at other formulas the same applies to Leopard.

Speed difference on an early 2009 8-core/16-thread Mac Pro:
<pre>
john --test --format=des
Benchmarking: Traditional DES [128/128 BS SSE2-16]... DONE
Many salts:	2764K c/s real, 2736K c/s virtual
Only one salt:	2646K c/s real, 2673K c/s virtual

john --test --format=des
Benchmarking: Traditional DES [128/128 BS SSE2-16]... (16xOMP) DONE
Many salts:	9538K c/s real, 763374 c/s virtual
Only one salt:	8585K c/s real, 740743 c/s virtual
</pre>

Don't confuse this with PR #19105 which enables OpenCL (GPU acceleration).