Fix for condd.test (conditional `+='):

Fix for condd.test (conditional `+='):
* automake.in (%appendvar): New.
(initialize_per_input): Clear it.
(macro_define): Handle += for variable defined in another condition.
* automake.texi (Conditional Sources): Use conditional += in
the example.
(General Operation, Conditionals): Remove note about broken +=.
* tests/cond21.test: New file.
* tests/condd.test: Also test conditional append to a _SOURCE
variable.  Create missing directories.
* tests/Makefile.am (TESTS): Add cond21.test.
(XFAILS): Remove condd.test.