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.

src/include/pg_config.h.in

@@ -851,6 +851,9 @@
 /* Define to select Win32-style shared memory. */
 #undef USE_WIN32_SHARED_MEMORY
 
+/* Define to 1 if `wcstombs_l' requires <xlocale.h>. */
+#undef WCSTOMBS_L_IN_XLOCALE
+
 /* Define WORDS_BIGENDIAN to 1 if your processor stores words with the most
    significant byte first (like Motorola and SPARC, unlike Intel). */
 #if defined AC_APPLE_UNIVERSAL_BUILD

src/include/pg_config.h.win32

@@ -657,6 +657,9 @@
 /* Define to select Win32-style semaphores. */
 #define USE_WIN32_SEMAPHORES 1
 
+/* Define to 1 if `wcstombs_l' requires <xlocale.h>. */
+/* #undef WCSTOMBS_L_IN_XLOCALE */
+
 /* Number of bits in a file offset, on hosts where this is settable. */
 /* #undef _FILE_OFFSET_BITS */
 

src/include/utils/pg_locale.h

@@ -13,7 +13,7 @@
 #define _PG_LOCALE_
 
 #include <locale.h>
-#ifdef LOCALE_T_IN_XLOCALE
+#if defined(LOCALE_T_IN_XLOCALE) || defined(WCSTOMBS_L_IN_XLOCALE)
 #include <xlocale.h>
 #endif