pgsql: Cope if platform declares mbstowcs_l(), but not locale_t, in <xl

Cope if platform declares mbstowcs_l(), but not locale_t, in <xlocale.h>.

Previously, we included <xlocale.h> only if necessary to get the definition
of type locale_t. According to notes in PGAC_TYPE_LOCALE_T, this is
important because on some versions of glibc that file supplies an
incompatible declaration of locale_t. (This info may be obsolete, because
on my RHEL6 box that seems to be the *only* definition of locale_t; but
there may still be glibc's in the wild for which it's a live concern.)

It turns out though that on FreeBSD and maybe other BSDen, you can get
locale_t from stdlib.h or locale.h but mbstowcs_l() and friends only from
<xlocale.h>. This was leaving us compiling calls to mbstowcs_l() and
friends with no visible prototype, which causes a warning and could
possibly cause actual trouble, since it's not declared to return int.

Hence, adjust the configure checks so that we'll include <xlocale.h>
either if it's necessary to get type locale_t or if it's necessary to
get a declaration of mbstowcs_l().

Report and patch by Aleksander Alekseev, somewhat whacked around by me.
Back-patch to all supported branches, since we have been using
mbstowcs_l() since 9.1.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/b4ce97aa5f603e039ed40af6a208c6f585d1ba9e

Modified Files
--------------
config/c-library.m4 | 32 +++++++++++++++++++++++++-
configure | 53 +++++++++++++++++++++++++++++++++++++++++++
configure.in | 1 +
src/include/pg_config.h.in | 3 +++
src/include/pg_config.h.win32 | 3 +++
src/include/utils/pg_locale.h | 2 +-
6 files changed, 92 insertions(+), 2 deletions(-)