One thing to pay attention to is that Snort 2.9.1 does require an additional dependency as noted in the release notes.
NOTE: Snort 2.9.1 requires pkg-config be installed for some
of its autoconf processing. See details below.
It is available for download here, if your system does not already have it.
Please see the Release Notes and ChangeLog for more details.
Please submit bugs, questions, and feedback to firstname.lastname@example.org.