configure - flex version test needs to be modified.

[MrBenGriffin]

found /usr/local/bin/flex, we'll use this. Use --with-flex to specify another.
checking flex version... configure: WARNING: Flex version 2.6.4 found.
    Version 2.5.31 or greater is required.  You can generally ignore this
    warning unless you need to regenerate psp_parser.c from psp_parse.l.
    If you do need regenerate psp_parser.c, use --with-flex to specify the
    location of the correct flex version. See the README for more information.

Flex version 2.6.4 found. Version 2.5.31 or greater is required.
Since I did maths at elementary school, I think I see a mistake here!