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.

include/libxml/parser.h

@@ -1613,6 +1613,7 @@ typedef enum {
     XML_WITH_ZLIB = 31,
     XML_WITH_ICU = 32,
     XML_WITH_LZMA = 33,
+    XML_WITH_RELAXNG = 34, /* since 2.14.0 */
     XML_WITH_NONE = 99999 /* just to be sure of allocation size */
 } xmlFeature;