* app/pixel_region.c, app/pixel_region.h: new function
	pixel_regions_process_parallel

	* app/paint_funcs.c: use pixel_regions_process_parallel in
	combine_regions.  fixed bug in combine_inten_a_and_inten_a_pixels

	* app/gimprc.c, app/gimprc.h, app/preferences_dialog.c: added
	num-processors variable

	* config.h.in, configure.in, acconfig.h: add configure option
 	--with-mp and define ENABLE_MP.