Do not raise "Unrecognized option -x" error if GMT is not compiled with OpenMP/GThreads support?

[seisman]

The -x common option is only available if GMT is compiled with OpenMP/GThreads support. If not, using -x gives the error: Unrecognized option -x. Users may be surprised why a script works on machine A but gives such an error on machine B. After reading the documentation or the man page, they may realize that GMT on machine B is not compiled with parallel support. Then users have to modify their working script unless they're unable to install another parallel-enabled GMT.

So, instead of giving the "Unrecognized option -x" error, we can make it always available.

• If GMT is compiled with parallel support, then everything works as before
• If GMT is NOT compiled with parallel support, then raise a WARNING and fallback to one core

In this way,

1. the warning message can clearly explain that the GMT is not parallel-enabled
2. users don't have to modify their working script if they don't care about parallel
3. the synopsis message can also be simplified (i.e., GMT_x_OPT, GMT_ADD_x_OPT)

[PaulWessel]

This is of course a recent change dealing with docs for -x and difference between OpenMP, Gthreads, and movie/batch. I would not care if we always parsed -x and if not compiled to support it we. Interested in fixing this by adding your warning instead?