* lib/Automake/ConditionalSet.pm (conds): Use value() to simplify.

* lib/Automake/ConditionalSet.pm (conds): Use value() to simplify.
(invert): Rewrite as a product-of-sums to sum-of-products converter.
(_multiply, multiply): New functions.
* lib/Automake/Conditional.pm (not): New function.
* lib/Automake/tests/ConditionalSet.pl (test_invert): Update.
* automake.in (condition_negate): Move ...
* lib/Automake/Conditional.pm (negate_condition): ... here.
Suggested by Raja R Harinath.