Do not bail out when pg_config is not found.

m4/ax_lib_postgresql.m4

@@ -80,7 +80,7 @@ AC_DEFUN([AX_LIB_POSTGRESQL],
         fi
 
         if test ! -x "$PG_CONFIG"; then
-            AC_MSG_ERROR([$PG_CONFIG does not exist or it is not an exectuable file])
+            dnl AC_MSG_ERROR([$PG_CONFIG does not exist or it is not an exectuable file])
             PG_CONFIG="no"
             found_postgresql="no"
         fi