Makes the build system to look for yajl using a macro file

Now searching for yajl using find_yajl.m4 macro file instead
of using pkg-config directly. If YAJL was not found or if it
was disabled in the configure phase, the code will be compiled
without JSON support.

apache2/modsecurity.c

@@ -256,8 +256,10 @@ static apr_status_t modsecurity_tx_cleanup(void *data) {
     /* XML processor cleanup. */
     if (msr->xml != NULL) xml_cleanup(msr);
 
+#ifdef WITH_YAJL
     /* JSON processor cleanup. */
     if (msr->json != NULL) json_cleanup(msr);
+#endif
 
     // TODO: Why do we ignore return code here?
     modsecurity_request_body_clear(msr, &my_error_msg);