With the different combinations of lablgtk2 and lablgtk3, the rule is somewhat
complex; besides, the `depends` should imply the GUI is always available;
finally, if there are nevertheless missing dependencies, the GUI will be
disabled by the configure script.

 - deprecation messages for alt[gr]-ergo and coq[ide]
 - require why3 >= 1.2.0 (includes conflicts with why3-base)
 - no more need for why3-coq