Assume that we have <wchar.h>.

Windows has this, and so do all other live platforms according to the
buildfarm; it's been required by POSIX since SUSv2.  So remove the
configure probe and tests of HAVE_WCHAR_H.

This is part of a series of commits to get rid of no-longer-relevant
configure checks and dead src/port/ code.  I'm committing them separately
to make it easier to back out individual changes if they prove less
portable than I expect.

Discussion: https://postgr.es/m/15379.1582221614@sss.pgh.pa.us

configure

@@ -12762,7 +12762,7 @@ $as_echo "#define HAVE_STDBOOL_H 1" >>confdefs.h
 fi
 
 
-for ac_header in atomic.h copyfile.h execinfo.h getopt.h ifaddrs.h langinfo.h mbarrier.h poll.h sys/epoll.h sys/event.h sys/ipc.h sys/prctl.h sys/procctl.h sys/pstat.h sys/resource.h sys/select.h sys/sem.h sys/shm.h sys/sockio.h sys/tas.h sys/un.h termios.h ucred.h wchar.h wctype.h
+for ac_header in atomic.h copyfile.h execinfo.h getopt.h ifaddrs.h langinfo.h mbarrier.h poll.h sys/epoll.h sys/event.h sys/ipc.h sys/prctl.h sys/procctl.h sys/pstat.h sys/resource.h sys/select.h sys/sem.h sys/shm.h sys/sockio.h sys/tas.h sys/un.h termios.h ucred.h wctype.h
 do :
   as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`
 ac_fn_c_check_header_mongrel "$LINENO" "$ac_header" "$as_ac_Header" "$ac_includes_default"

configure.in

@@ -1300,7 +1300,6 @@ AC_CHECK_HEADERS(m4_normalize([
 	sys/un.h
 	termios.h
 	ucred.h
-	wchar.h
 	wctype.h
 ]))
 

src/backend/utils/adt/formatting.c

@@ -72,11 +72,9 @@
 
 /*
  * towlower() and friends should be in <wctype.h>, but some pre-C99 systems
- * declare them in <wchar.h>.
+ * declare them in <wchar.h>, so include that too.
  */
-#ifdef HAVE_WCHAR_H
 #include <wchar.h>
-#endif
 #ifdef HAVE_WCTYPE_H
 #include <wctype.h>
 #endif

src/include/pg_config.h.in

@@ -677,9 +677,6 @@
 /* Define to 1 if you have the <uuid/uuid.h> header file. */
 #undef HAVE_UUID_UUID_H
 
-/* Define to 1 if you have the <wchar.h> header file. */
-#undef HAVE_WCHAR_H
-
 /* Define to 1 if you have the `wcstombs_l' function. */
 #undef HAVE_WCSTOMBS_L
 

src/include/regex/regcustom.h

@@ -43,11 +43,9 @@
 
 /*
  * towlower() and friends should be in <wctype.h>, but some pre-C99 systems
- * declare them in <wchar.h>.
+ * declare them in <wchar.h>, so include that too.
  */
-#ifdef HAVE_WCHAR_H
 #include <wchar.h>
-#endif
 #ifdef HAVE_WCTYPE_H
 #include <wctype.h>
 #endif

src/include/tsearch/ts_locale.h

@@ -20,11 +20,9 @@
 
 /*
  * towlower() and friends should be in <wctype.h>, but some pre-C99 systems
- * declare them in <wchar.h>.
+ * declare them in <wchar.h>, so include that too.
  */
-#ifdef HAVE_WCHAR_H
 #include <wchar.h>
-#endif
 #ifdef HAVE_WCTYPE_H
 #include <wctype.h>
 #endif

src/tools/msvc/Solution.pm

@@ -396,7 +396,6 @@ sub GenerateFiles
 		HAVE_UUID_H                              => undef,
 		HAVE_UUID_UUID_H                         => undef,
 		HAVE_WINLDAP_H                           => undef,
-		HAVE_WCHAR_H                             => 1,
 		HAVE_WCSTOMBS_L                          => 1,
 		HAVE_WCTYPE_H                            => 1,
 		HAVE_X509_GET_SIGNATURE_NID              => 1,