Fix for PR automake/352:

Fix for PR automake/352:
* automake.in (transform_variable_recursively): If the destination
and the source variables are the same, delete the source variable
before defining the destination variable.
(handle_programs): Strip any $(EXEEXT) suffix from $one_file,
or this will confuse handle_source_transform.
(append_exeext): Rewrite using transform_variable_recursively.
* tests/cond29.test, tests/cond30.test: New files.
* tests/exeext.test: Run ./configure and make, this uncovered
the above handle_programs issue.
* tests/Makefile.am (TESTS): Add cond29.test and cond20.test.