* automake.in (&variable_output): Admit a list of @CONDS.

* automake.in (&variable_output): Admit a list of @CONDS.
(&variable_pretty_output): New.
(&define_pretty_variable): Use it.
(&read_am_file, &file_contents_internal): Prepend a separator to
$am_vars only if there is none yet.
(&file_contents_internal): Rename $separator as $spacing to
harmonize with &read_am_file.