Support building with Visual Studio 2015

Adjust the way we detect the locale. As a result the minumum Windows
version supported by VS2015 and later is Windows Vista. Add some tweaks
to remove new compiler warnings. Remove documentation references to the
now obsolete msysGit.

Michael Paquier, somewhat edited by me, reviewed by Christian Ullrich.

Backpatch to 9.5

src/port/win32env.c

@@ -69,6 +69,9 @@ pgwin32_putenv(const char *envval)
 		{
 			"msvcr120", 0, NULL
 		},						/* Visual Studio 2013 */
+		{
+			"urctbase", 0, NULL
+		},						/* Visual Studio 2015 and later */
 		{
 			NULL, 0, NULL
 		}