Eliminate too-expansive "clean" rule kluge by requiring fixed Automake.

The NEWS for Automake 1.10.1 mentions the following bug fix:

  - All directories `.libs'/`_libs' used by libtool are cleaned now,
    not only those in which libraries are built.

This describes the situation for the kluged "clean" rule in
tests/automake.mk, so by requiring Automake 1.10.1 or later we can also
eliminate the clean rule.