* automake.in (&variable_dump): If a variable is undefined, say

* automake.in (&variable_dump): If a variable is undefined, say
it.
(&check_ambiguous_conditional): Give finer error messages.
(&macro_define): Check that a `+=' variable is not set with `='.
Check for ambiguous definitions each time you _set_ (even with `+=')
a variable.
(&read_main_am_file): Perform a deep copy of %conditional.