intl: reintroduce unintentionally disabled optimization

HAVE_BUILTIN_EXPECT macro was removed by commit glibc-2.14-280-g3ce1f29,
but then its use was unintentionally reintroduced during merge with GNU
gettext 0.19.3 by commit glibc-2.20-324-g6d24885, effectively disabling
all optimization based on __builtin_expect.  As intl files are also part
of GNU gettext, HAVE_BUILTIN_EXPECT macro cannot be removed, so define
it unconditionally in config.h.in instead.

[BZ #19512]
* config.h.in (HAVE_BUILTIN_EXPECT): New macro.