Disable documentation when hevea is not available. (Fix for bug #16314.)
Disable documentation when hevea is not available. (Fix for bug #16314.)

jpdeplaix/why3,jpdeplaix/why3,ssaavedra/why3,ssaavedra/why3,jpdeplaix/why3,ssaavedra/why3,douglas-larocca/why3,douglas-larocca/why3,douglas-larocca/why3