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
Only use __underlying_type for gcc >= 4.7. Didn't exist previously.

Change-Id: I45cd90ddb201f4d0c72c7722881f77f90167ff23
Merge pull request #132 from awong-dev/underlying-type

Underlying type