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.