Build: Looking for YAJL's LD flags instead of CFLAGS

Original author: @LinuxJedi. Reported at #1077.

Author: zimmerle

build/yajl.m4

@@ -85,7 +85,7 @@ else
 #    fi
 fi
 
-if test -z "${YAJL_CFLAGS}"; then
+if test -z "${YAJL_LDADD}"; then
     if test -z "${YAJL_MANDATORY}"; then
         if test -z "${YAJL_DISABLED}"; then
             AC_MSG_NOTICE([YAJL library was not found])