* automake.in (conditional_true_when): Return false if $WHEN == FALSE.

* automake.in (conditional_true_when): Return false if $WHEN == FALSE.
(conditional_is_redundant): Simplify.
(conditional_implies_one_of,
variable_not_always_defined_in_cond): New functions
(macro_define): Reject appends if the variable is not defined in
all conditions where `+=' applies.
(invert_conditions): Rewrite.  Before this patch,
invert_conditions("A_TRUE B_TRUE", "A_TRUE B_FALSE", "A_FALSE")
would return ("A_FALSE B_TRUE", "A_FALSE B_TRUE"), which seems
wrong (these conditions implies "A_FALSE").  Now it outputs (),
which just means the input conditions cover all cases.
(variable_conditions_permutations): Never output FALSE conditions.
* tests/pluseq2.test, tests/pluseq3.test: Define data_DATA
in the CHECK_FALSE condition to fix the test.
* tests/pluseq5.test: Actually check the diagnostic.
* tests/pluseq9.test: New file.
* tests/Makefile.am (TESTS): Add pluseq9.test.