* lib/Automake/Conditional.pm: Add reference to ConditionalSet.pm.

* lib/Automake/Conditional.pm: Add reference to ConditionalSet.pm.
* lib/Automake/ConditionalSet.pm: New file.
* lib/Automake/Makefile.am (dist_perllib_DATA): Add ConditionalSet.pm.
* automake.in: Use ConditionalSet.
(by_condition, invert_conditions, variable_conditions_permutations):
Remove, now defined as Automake::ConditionalSet::by_condition,
Automake::ConditionalSet::invert, and
Automake::ConditionalSet::permutations.
(variable_sorted_conditions): Remove.
(msg_var, msg_target handle_source_transform, handle_lib_objects,
conditional_ambiguous_p, variable_not_always_defined_in_cond,
macro_define, macro_dump, variable_defined,
variable_conditions_recursive, variable_conditions,
target_conditions, variable_conditionally_defined,
variable_conditions_recursive_sub,
check_variable_defined_unconditionally, variable_value_as_list,
variable_value_as_list_recursive_worker, variable_output,
variable_pretty_output, rule_define, append_exeext,
am_install_var, require_variables_for_macro): Adjust to
use ConditionalSet.