Add macros for diagnostic control, use them in locale/weightwc.h

[Roland McGrath]

Thanks very much for tracking down that change!  That's exactly what we'd
like to see done instead of warning suppression whenever it's possible.
The only thing I'd like you to do differently is to put more explanation in
the code itself rather than only in the ChangeLog.  e.g.

	/* It's important that this be a scalar variable rather than a
	   one-element array, because GCC (at least 4.9.2 -O2 on x86-64)
	   can be confused by the array and diagnose a "used initialized"
	   in a dead branch in the findidx function.  */


Thanks,
Roland