Diagnose target clashes, for PR automake/344:

Diagnose target clashes, for PR automake/344:
* automake.in (%targets): Record conditionals for definitions.
(%target_conditional): Remove (obsoleted by %targets).
(%target_source, %target_owner): New hashes.
(TARGET_AUTOMAKE, TARGET_USER): New constants.
(initialize_per_input): Adjust to reset new variables.
(err_cond_target, msg_cond_target): New functions.
(msg_target): Adjust usage of %targets.
(conditional_ambiguous_p): Take a list of conditional to check
as a third parameter, so this can be used for other things that
variables.
(handle_lib_objects_cond): Adjust conditional_ambiguous_p usage.
(variable_defined): Restrict the target-with-same-name check
to user targets.
(rule_define): Add the $SOURCE argument, and take $OWNER instead
of $IS_AM.  Diagnose target clashes (including ambugious
conditionals).  Return a list of conditions where the rule should
be defined instead of a boolean.  Fill %target_source and
%target_owner.
(target_define): Use `exists', not `defined'.
(read_am_file): Adjust the call to rule_define.
(file_contents_internal): Add more FIXMEs.  Simplify my moving
and documenting the "define rules in undefined conditions" to
rule_define.
* tests/Makefile.am (XFAIL_TESTS): Add specflags7.test and
specflags8.test.