Fixes utilities/meta/meta.trans/meta.trans.other/underlying_type.pass.cpp

std::underlying_type requires compiler frontend support. Our config file
wasn't providing one for gcc. Easy fix. Might need a version
restriction though since I don't think it was available in gcc 4.3 and earlier.

Change-Id: I13743c7461c2f150828a58fde7c85f9011560f00