Build gcc.debug using -Og flag

Since gcc 4.8 is required anyways, it might be nice to use its features.

Intro to feature (second bullet point):
https://gcc.gnu.org/gcc-4.8/changes.html

-g (line 328) is still needed:
http://stackoverflow.com/questions/12970596/gcc-4-8-does-og-imply-g
Exactly.