build: don't compile debug build with -Og

It's not supported by clang and commit e67542ae ("build: disable -Og
when building with clang") is not sufficient because the configure
script no longer writes the 'clang' variable to common.gypi.

I could fix the configure script but I don't care enough actually do
so.  A fixed configure script won't help anyway when the compiler is
overridden through the CXX environment variable at compile time.

PR-URL: https://github.com/iojs/io.js/pull/1611


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