Fix PR automake/326:
* automake.in (define_objects_from_sources): Calculate the result in
all conditions before passing this to subobjname to pick a name to
store the result in.
(subobjname): Expect a list of condition/value pairs as input.
Combine this list and use it as the key to determine the variable name.
* test/cond22.test: New file.
* tests/Makefile.am (TESTS): Add cond21.test