All supported systems have locale_t.

locale_t is defined by POSIX.1-2008 and SUSv4, and available on all
targeted systems.  For Windows, win32_port.h redirects to a partial
implementation called _locale_t.  We can now remove a lot of
compile-time tests for HAVE_LOCALE_T, and associated comments and dead
code branches that were needed for older computers.

Since configure + MinGW builds didn't detect locale_t but now we assume
that all systems have it, further inconsistencies among the 3 Windows build
systems were revealed.  With this commit, we no longer define
HAVE_WCSTOMBS_L and HAVE_MBSTOWCS_L on any Windows build system, but
we have logic to deal with that so that replacements are available where
appropriate.

Reviewed-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Tristan Partin <tristan@neon.tech>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CA%2BhUKGLg7_T2GKwZFAkEf0V7vbnur-NfCjZPKZb%3DZfAXSV1ORw%40mail.gmail.com

src/include/utils/pg_locale.h

@@ -67,9 +67,7 @@ extern void cache_locale_time(void);
 
 
 /*
- * We define our own wrapper around locale_t so we can keep the same
- * function signatures for all builds, while not having to create a
- * fake version of the standard type locale_t in the global namespace.
+ * We use a discriminated union to hold either a locale_t or an ICU collator.
  * pg_locale_t is occasionally checked for truth, so make it a pointer.
  */
 struct pg_locale_struct
@@ -78,9 +76,7 @@ struct pg_locale_struct
 	bool		deterministic;
 	union
 	{
-#ifdef HAVE_LOCALE_T
 		locale_t	lt;
-#endif
 #ifdef USE_ICU
 		struct
 		{
@@ -88,7 +84,6 @@ struct pg_locale_struct
 			UCollator  *ucol;
 		}			icu;
 #endif
-		int			dummy;		/* in case we have neither LOCALE_T nor ICU */
 	}			info;
 };