configure.ac should check for a jansson 2.3 or above library

[moreandres]

After seeing this error:

$ sudo mqexec /etc/nagios/nagmq.config -v
Error getting config Unexpected format character '?'

I've found that my default Ubuntu repositories didn't have a recent enough version of lib jansson.

Ubuntu 12.04.4 LTS 2.2.1-1

According to the documentation the '?' extension for optional matching was introduced with version 2.3, I've updated to 2.5 at mqbroker executed.

New in version 2.3: Any s representing a key may be suffixed with a ? to make the key optional. If the key is not found, nothing is extracted. See below for an example.

[jbreams]

I'll add pkg-config to the jansson check, and that should fix this.