build: disable -Og when building with clang

clang does not yet support -Og, fall back to -O0.

See: https://llvm.org/bugs/show_bug.cgi?id=20765
Fixes: https://github.com/iojs/io.js/issues/1608
PR-URL: https://github.com/iojs/io.js/pull/1609


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>