automake: fix regression due to de-ansification support removal

automake: fix regression due to de-ansification support removal

The last change `v1.11-947-g136b489' removed code that automake
was using to decide whether binary objects were built by the
generated Makefile.in, so that it could avoid to emit unneeded
code when this was not the case.  Re-introduce such code in a
less-obfuscated form, and add a test to ensure we don't regress
again.

* automake.in ($must_handle_compiled_objects): New global
variable, telling whether the generated Makefile has to build
compiled objects.
(initialize_per_input): Reset it.
(handle_programs, handle_libraries, handle_ltlibraries): Set
it to a true value when required.
(handle_compile): Don't generate any code if the variable
`$must_handle_compiled_objects' is not set to a true value.
* tests/no-extra-makefile-code.test: New test.
* tests/Makefile.am (TESTS): Add it.