[PATCH] mention disabling GCC built-ins for customization

[Martin Sebor]

The GCC sprintf optimization introduced in GCC 7 relies on
the conformance of the library implementation of formatted
I/O functions.  When a sprintf customization changes
the effects of the function in a way that's observable by
a strictly conforming program it might interfere with
the optimization.

Since this may not be obvious to users the attached patch
adds a couple of sentences to the manual to make it clear
that the sprintf built-in handling needs to be disabled
when customizing the function.

Martin
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