build: Add separate configuration option for RELAX NG

Support for RELAX NG used to be enabled together with XML Schema support
(--with-schemas). Now there's a separate option and a new feature macro
LIBXML_RELAXNG_ENABLED.

parser.c

@@ -657,6 +657,12 @@ xmlHasFeature(xmlFeature feature)
             return(1);
 #else
             return(0);
+#endif
+        case XML_WITH_RELAXNG:
+#ifdef LIBXML_RELAXNG_ENABLED
+            return(1);
+#else
+            return(0);
 #endif
         case XML_WITH_SCHEMAS:
 #ifdef LIBXML_SCHEMAS_ENABLED