dist: separate auxiliary file instantiation from DIST_COMMON update

dist: separate auxiliary file instantiation from DIST_COMMON update

This change simplifies the automake internals dealing with the
checking, copying and distributing of required auxiliary files.
With this change, a required auxiliary file is *unconditionally*
added to the contents of the DIST_COMMON variable in the generated
Makefile.in, before checking whether it exists, or trying to copy
it (if `--add-missing' is in use).  This shouldn't be a problem,
since if the checking or copying of the file fails, automake will
bail out, the Makefile.in won't be created, and thus its content
will not matter.  OK, this is not completely true when threaded
automake is in use, but then, such a situation was also possible
before this patch, so no regression here, which is enough for us.

This change is the first of a series of steps aimed at fixing
the regression introduced in threaded automake usage by commit
`v1.11-1219-g326ecba'.

* automake.in (require_file_internal): Add the required file to
DIST_COMMON unconditionally.
* tests/missing-auxfile-stops-makefiles-creation.test: New test.
* tests/Makefile.am (TESTS): Add it.