CLP installed by default instead of Soplex if --with-soplex is missing about ibex-lib

[Jordan08]

For now, if no LP solver is specifyed, we install CLP.
But you're right we should check if soplex or cplex is available before installing CLP.

[gchabert]

I'm fine with CLP as long as it can be installed properly on all platforms (as soplex up to now).
In fact I even prefer a library to be automatically installed from the bundle (in case of missing explicit option like --with-soplex) than a script looking to a particular library on a parent directory.
Note: we also need to update the installation doc on this point.

[Jordan08]

J'ai remis un peu d'ordre dans le wscript
Maintenant, si on ne précise pas le solver, le waf cherche les solver lineaire existant dans les chemins de base. Si il en trouve un, il l'utilise sinon il installe CLP.

J'ai rajouté également l'option "--without-lp" si on veut installer IBEX sans solver lineaire.