Units: don't use subshell  when checking features and languages

If checking them in subshells, their result are not reflected to
the parent shell.

Issue: #60
Reported-by: Frank Fesevur <ffes@users.sourceforge.net>
Signed-off-by: Masatake YAMATO <yamato@redhat.com>
testing.mak: add clean target

Issue: #60
Suggested-by: Frank Fesevur <ffes@users.sourceforge.net>
Signed-off-by: Masatake YAMATO <yamato@redhat.com>