Clean target must not fail

The Debian package no more builts properly, because it runs "make clean" always before building, even if no build happened before:

     dpkg-source -i --before-build aha
    dpkg-buildpackage: host architecture amd64
     fakeroot debian/rules clean
    dh clean
       dh_testdir
       dh_auto_clean
    make[1]: Entering directory '/home/abe/aha/aha'
    rm aha
    rm: cannot remove ‘aha’: No such file or directory
    Makefile:16: recipe for target 'clean' failed
    make[1]: *** [clean] Error 1
    make[1]: Leaving directory '/home/abe/aha/aha'
    dh_auto_clean: make -j1 clean returned exit code 2
    debian/rules:8: recipe for target 'clean' failed
    make: *** [clean] Error 2
    dpkg-buildpackage: error: fakeroot debian/rules clean gave error exit status 2

This patch fixes the issue.