Assume setlocale exists

Assume setlocale exists

Like locale.h, it was standardized by C89, is universally
available now, and some code already assumes it.
* configure.ac: Do not check for setlocale.
* src/emacs.c (setlocale) [!HAVE_SETLOCALE]: Remove.
(fixup_locale, synchronize_locale, Vprevious_system_time_locale)
(synchronize_system_time_locale): Define even if !HAVE_SETLOCALE.
* src/sysdep.c (emacs_setlocale): Simplify by assuming HAVE_SETLOCALE.

(cherry picked from commit 69374fbe827b83c49eff084de8635220bd6d3ccf)