* lib/Automake/Variable.pm (define, _new): Remember the helper

* lib/Automake/Variable.pm (define, _new): Remember the helper
variable created for the last conditional += on each variable, and
only append further += in the same condition to this last helper
variable, not to older helper variables.  This way the order of
the items appended to the variable is preserved.
* tests/cond21.test: Adjust.
* tests/cond38.test: New file.
* tests/Makefile.am (TESTS): Add cond38.test.
Report from Ed Hartnett.