Add macro PGWARNING, and make PGERROR available on all platforms.

We'd previously noted the need for coping with Windows headers
that provide some other definition of macro "ERROR" than elog.h
does.  It turns out that R also wants to define ERROR, and
WARNING too.  PL/R has been working around this in a hacky way
that broke when we recently changed the numeric value of ERROR.
To let them have a more future-proof solution, provide an
alternate macro PGWARNING for WARNING, and make PGERROR visible
always, not only when #ifdef WIN32.

Discussion: https://postgr.es/m/CADK3HHK6iMChd1yoOqssxBn5Z14Zar8Ztr3G-N_fuG7F8YTP3w@mail.gmail.com

src/include/utils/elog.h

@@ -40,20 +40,22 @@
 #define WARNING		19			/* Warnings.  NOTICE is for expected messages
 								 * like implicit sequence creation by SERIAL.
 								 * WARNING is for unexpected messages. */
+#define PGWARNING	19			/* Must equal WARNING; see NOTE below. */
 #define WARNING_CLIENT_ONLY	20	/* Warnings to be sent to client as usual, but
 								 * never to the server log. */
 #define ERROR		21			/* user error - abort transaction; return to
 								 * known state */
-/* Save ERROR value in PGERROR so it can be restored when Win32 includes
- * modify it.  We have to use a constant rather than ERROR because macros
- * are expanded only when referenced outside macros.
- */
-#ifdef WIN32
-#define PGERROR		21
-#endif
+#define PGERROR		21			/* Must equal ERROR; see NOTE below. */
 #define FATAL		22			/* fatal error - abort process */
 #define PANIC		23			/* take down the other backends with me */
 
+/*
+ * NOTE: the alternate names PGWARNING and PGERROR are useful for dealing
+ * with third-party headers that make other definitions of WARNING and/or
+ * ERROR.  One can, for example, re-define ERROR as PGERROR after including
+ * such a header.
+ */
+
 
 /* macros for representing SQLSTATE strings compactly */
 #define PGSIXBIT(ch)	(((ch) - '0') & 0x3F)