Correctly determine language for more -std= flags

In particular we now support e.g. gnu++1z for C++ as well as gnu99 or
iso9899:1999 and similar for C. This means that the -imacros file will
no longer define __cplusplus when parsing C code.

We also print a warning message now we couldn't determine the language
from the -std= flag

My files that are compiled with -std==iso9899:1999 show errors about
nullptr not being known. This happened before because __cplusplus was
defined and NULL was therefore defined to nullptr