Add ebuild and compiler override for trace flags

Not sure if this is useful to anyone but me. 

This is my current ebuild (gentoo package description) for Granary. The change in globals.h lets me set GR_EXTRA_C{C,XX}_FLAGS to turn trace on or off. Lets me quickly rebuild with trace on or off from git head rather and building in a git checkout and modifying the source there. 

Obviously pretty specific to my workflow. Figure I would open a PR cause it's just free code :)