Tidy and fix clean and distclean rules for old testsuite.

Tidy and fix clean and distclean rules for old testsuite.

* Makefile.am (clean-local-legacy): Use $(CONF_SUBDIRS) instead
of hard-coding the list of test directories.  Use
$(AM_MAKEFLAGS).
(fake-distclean-legacy): New phony rule, to create fake Makefile
files if needed so that the automake-generated
distclean-recursive rule can work properly.
(distclean_recursive): New helper variable.
(distclean-recursive): Depend on fake-distclean-legacy.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>