Support autoclean for files outside folder

[Matthew D. Scholefield]

I have an install() target that copies files to /usr/local/include etc. but running autoclean() doesn't remove these files. The .deps file shows:

"cp libfann.so /usr/local/lib/": {
    "libfann.so": "input-86f85e8d508e0b4a4b02b9657756d9b5"
},