Update "pg_regress --no-locale" for Darwin and Windows.

Commit 894459e59f revealed this option to
be broken for NLS builds on Darwin, but "make -C contrib/unaccent check"
and the buildfarm client rely on it.  Fix that configuration by
redefining the option to imply LANG=C on Darwin.  In passing, use LANG=C
instead of LANG=en on Windows; since only postmaster startup uses that
value, testers are unlikely to notice the change.  Back-patch to 9.0,
like the predecessor commit.

src/test/regress/pg_regress.c

@@ -790,9 +790,17 @@ initialize_environment(void)
 		unsetenv("LC_NUMERIC");
 		unsetenv("LC_TIME");
 		unsetenv("LANG");
-		/* On Windows the default locale cannot be English, so force it */
-#if defined(WIN32) || defined(__CYGWIN__)
-		putenv("LANG=en");
+
+		/*
+		 * Most platforms have adopted the POSIX locale as their
+		 * implementation-defined default locale.  Exceptions include native
+		 * Windows, Darwin with --enable-nls, and Cygwin with --enable-nls.
+		 * (Use of --enable-nls matters because libintl replaces setlocale().)
+		 * Also, PostgreSQL does not support Darwin with locale environment
+		 * variables unset; see PostmasterMain().
+		 */
+#if defined(WIN32) || defined(__CYGWIN__) || defined(__darwin__)
+		putenv("LANG=C");
 #endif
 	}