Clean target must not fail if there's nothing to clean

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.