thanks to Frank Seesink on MODSKINNY-6

declaring some variables "extern" thanks to Peter Olsson

Fix MODSKINNY-6: Build still fails on OS X (even after last changes)

It provides better detection than mod_vmd but is a little more CPU intensive than mod_vmd.

Perhaps someone would like to CUDAfy this code?

Resolve (shouldn't) MODSKINNY-5: Compiling mod_skinny on Mac OS X 10.6.3 fails
with error message about format mismatch of 'long unsigned int'