This can be helpful in porting to compilers like Oracle Developer
Studio that support some but not all GCC attributes.
* lib/wordsplit.c (FALLTHROUGH): Remove; now done by attribute.h.
* lib/wordsplit.h (__WORDSPLIT_ATTRIBUTE_FORMAT): Remove;
all uses replaced by ATTRIBUTE_FORMAT.
* lib/wordsplit.h, src/buffer.c, src/common.h, src/compare.c:
* src/sparse.c, src/system.c, src/xheader.c:
Prefer ATTRIBUTE_FORMAT, MAYBE_UNUSED, _Noreturn, etc. to
__attribute__.
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