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/backend/utils/adt/like.c

@@ -95,10 +95,8 @@ SB_lower_char(unsigned char c, pg_locale_t locale, bool locale_is_c)
 {
 	if (locale_is_c)
 		return pg_ascii_tolower(c);
-#ifdef HAVE_LOCALE_T
 	else if (locale)
 		return tolower_l(c, locale->info.lt);
-#endif
 	else
 		return pg_tolower(c);
 }