Add optional OpenMP dependency

Per @Jehan 's comment on !749, this merge request attempts to add support for an optional OpenMP dependency in both build systems. I have built GIMP with this patch via both build systems; however, this is my first time adding to the build rules. If I've missed anything or set anything up incorrectly, please let me know.

Currently, the only part of GIMP that's coded to use OpenMP is the DDS plug-in in mipmap.c and dxt.c. I have tested it with several images and been abled to import/export with a mipmap. @Wormnest, since you've done a lot of work with DDS, would you be willing to take a look at it when you have time and see if there's any changes in performance or output with OpenMP enabled?