cmake in minisatid: empty include_directory

[Broes De Cat]

In some setups, the flex include directory is empty in the minisatid src cmake file. This should be no problem, but cmake stops on it with an error, so an include guard should be added.

[Bart Bogaerts]

Sometimes, running cmake twice fixes this problem

[Broes De Cat]

Fixes #600.

→ <<cset 97b687ebcae0>>