Fix PR automake/315:

Fix PR automake/315:
* automake.in (subobjname): Rewrite to generate variable name
unique for each content.
(%substnums): Remove.
(%subobjvar): New hash.
(initialize_per_input): Clear %subobjvar.
(define_objects_from_sources): Return the name of the variable
defined, in addition to the linker.  Call subobjname only once
the content of the variable to define is known.
(handle_source_transform): Adjust call to define_objects_from_sources.
* tests/specflags8.test: Mention PR 315.
* tests/subobjname.test: New file.
* tests/Makefile.am (XFAIL_TESTS): Remove specflags8.test.
(TESTS): Add subobjname.test.