Add more specific flags for misc GNU extensions.

This adds the following as subgroups of -Wgnu: -Wgnu-alignof-expression,
-Wgnu-case-range, -Wgnu-complex-integer, -Wgnu-conditional-omitted-operand,
-Wgnu-empty-initializer, -Wgnu-label-as-value, -Wgnu-local-label,
and -Wgnu-statement-expression,

Patch by Peter Lewis.
Remove Extension warning for GNU local labels.

We generally don't warn about extensions involving keywords reserved
for the implementation, so we shouldn't warn here either: the
standard doesn't require it, and it doesn't provide useful information
to the user.