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/msc_json.c

@@ -14,6 +14,8 @@
 
 #include "msc_json.h"
 
+#ifdef WITH_YAJL
+
 int json_add_argument(modsec_rec *msr, const char *value, unsigned length)
 {
     msc_arg *arg = (msc_arg *) NULL;
@@ -311,3 +313,4 @@ apr_status_t json_cleanup(modsec_rec *msr) {
     return 1;
 }
 
+#endif