Properly implement OpenMP support

Increases performance on multi-core systems, mostly notable in cubic
resizing. No effect on single-core systems. You'll need OpenMP support
in GCC to use this (which is currently not the case in OE so you won't
see any improvement unless you built your own compiler).

On an 9x00 "grab -b" takes 2.23s with OpenMP, 2.80s without.
Fix build fail without openmp

Diagnostic "fail without it" wasn't supposed to go public. There's about
one location in the world where _OPENMP gets defined currently...
Merge branch 'master' of git://openpli.git.sourceforge.net/gitroot/openpli/aio-grab

Conflicts:
	main.c