Add caveat for GLPK-4.52 and Octave-3.6.4

`./configure` produces a warning:

    configure: WARNING: GLPK library not found. The glpk function for solving linear programs will be disabled.

This issue is already reported here:
https://bugs.archlinux.org/task/36835

> Octave cannot use the current version of GLPK, its API has apparently changed.

As mentioned in the ArchLinux ticket, the GLPK API has changed.

I have tested this with a manually compiled `glpk-4.52` and `octave-3.6.4`

I also tested a compilation of stable `glpk-4.52` and this revision of `octave`:
http://hg.savannah.gnu.org/hgweb/octave/rev/7c48493d98e1, which was successful in linking against `glpk`