Change the handling of conditionals: instead of using an ad-hoc

Change the handling of conditionals: instead of using an ad-hoc
encoding to store a hash in a string, use hashes.
* automake.in (&conditional_dump): New.
(&check_ambiguous_conditional, &variable_defined)
(&variable_conditions_sub, &variable_value_as_list_worker)
(&define_variable, read_am_file, &read_main_am_file): Be sure to
handle `$conditional{$vars}' as a hash instead of a plain string.