build: turn on debug-safe optimizations with -Og

The resulting binary is still easy to inspect in gdb but is not as dog
slow.  The 'parallel' test suite, although it takes several minutes to
complete, now finishes without any tests timing out.

PR-URL: https://github.com/iojs/io.js/pull/1569


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>