Add PQlibVersion() function to libpq

This function is like the PQserverVersion() function except
it returns the version of libpq, making it possible for a client
program or driver to determine which version of libpq is in
use at runtime, and not just at link time.

Suggested by Harald Armin Massa and several others.

src/interfaces/libpq/libpq-fe.h

@@ -547,6 +547,9 @@ extern int	lo_export(PGconn *conn, Oid lobjId, const char *filename);
 
 /* === in fe-misc.c === */
 
+/* Get the version of the libpq library in use */
+extern int	PQlibVersion(void);
+
 /* Determine length of multibyte encoded char at *s */
 extern int	PQmblen(const char *s, int encoding);