too-aggressive clean rule

[Ben Pfaff]

Commit 99bafccc08919a5d640cb35962c4d730a2cad3a1 "Fix make
distcheck" introduces the following:

    # This seems to be necessary as the libtool artifacts aren't removed
    # Automake bug ?
    tests_data_datasheet_test_CLEAN:
            $(RM) -r tests/data

There are a couple of problems with this.  First, $(RM) isn't
defined on every system, and so sometimes it expands to the null
string.  I'm not sure where it's defined, in fact, but Automake
itself just uses "rm" all over, so I'm sure that it's OK to use
plain "rm".  Second, if you run configure" in the source
directory then this deletes datasheet-test.c and
datasheet-test.sh, which is obviously a bad idea.  Finally, I
think that -f is warranted in case the files in question haven't
been built.

I see a few other uses of $(RM) elsewhere, too:

    perl-module/automake.mk:        $(RM) $@
    perl-module/automake.mk:          $(RM) $(module_sources) ; \
    perl-module/automake.mk:        $(RM) perl-module/Makefile.old
    po/automake.mk:   $(RM) 
$(DESTDIR)$(prefix)/share/locale/$$lang/LC_MESSAGES/$(DOMAIN).mo ; \
    src/ui/gui/automake.mk:          $(RM) 
$(themedir)/$$size/$(context)/psppicon.png ; \
    tests/automake.mk:      $(RM) -r tests/data

Jason: This probably explains the problem you were seeing.
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