pgsql: Simplify checking for xlocale.h

Simplify checking for xlocale.h

Instead of XXX_IN_XLOCALE_H for several features XXX, let's just
include <xlocale.h> if HAVE_XLOCALE_H. The reason for the extra
complication was apparently that some old glibc systems also had an
<xlocale.h>, and you weren't supposed to include it directly, but it's
gone now (as far as I can tell it was harmless to do so anyway).

Author: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Discussion: https://postgr.es/m/CWZBBRR6YA8D.8EHMDRGLCKCD%40neon.tech

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9c2a6c5a5f4b94a93120009e623ae8bd153e6dbb

Modified Files
--------------
config/c-library.m4 | 55 --------------
configure | 104 +--------------------------
configure.ac | 4 +-
meson.build | 33 +--------
src/include/c.h | 3 +
src/include/pg_config.h.in | 9 +--
src/include/utils/pg_locale.h | 3 -
src/interfaces/ecpg/ecpglib/ecpglib_extern.h | 3 -
src/port/chklocale.c | 4 --
9 files changed, 9 insertions(+), 209 deletions(-)