zungmann's fixes to compile simulator on a Mac

Compiling for Mac needs some customizations noticed by zungmann in
the reprap forum, here:

http://forums.reprap.org/read.php?147,33082,279050#msg-279050

  * .bss section syntax is different
  * on_exit is not supported (but atexit is)
  * clock_gettime is not supported
Merge pull request #71 from Traumflug/mac-simulator

zungmann's fixes to compile simulator on a Mac