Clean up GNU-isms in base?

[Rares Aioanei]

While compiling world with extra warnings I noticed compiler messages
about GNU extensions being used in the code, for example the use of typeof() or
warnings like this : "/usr/src/lib/libpmc/pmu-events/json.h:22:35:
warning: token pasting of ',' and __VA_ARGS__ is a GNU extension
[-Wgnu-zero-variadic-macro-arguments]". Question is : is there an
interest in doing a "clean-up" of such constructs in the code?