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.

threads.c

@@ -19,9 +19,11 @@
 #ifdef LIBXML_CATALOG_ENABLED
 #include <libxml/catalog.h>
 #endif
+#ifdef LIBXML_RELAXNG_ENABLED
+#include <libxml/relaxng.h>
+#endif
 #ifdef LIBXML_SCHEMAS_ENABLED
 #include <libxml/xmlschemastypes.h>
-#include <libxml/relaxng.h>
 #endif
 
 #if defined(SOLARIS)
@@ -494,6 +496,8 @@ xmlCleanupParser(void) {
 #endif
 #ifdef LIBXML_SCHEMAS_ENABLED
     xmlSchemaCleanupTypes();
+#endif
+#ifdef LIBXML_RELAXNG_ENABLED
     xmlRelaxNGCleanupTypes();
 #endif