When running tests, make sure that the new bnfc is used

even if an old version is already installed on the system
Adds a new system test for haskell with GADT

Exibits bug #32
foo
Apply patch from Alex Rozenshteyn to fix the gadt backend
Disable the gadt tests by default

See bug #34 for more info
Merge pull request #35 from gdetrez/master

Fixes bug #32