buildEnv: Check the content of colliding paths.

Originally wanted to include ignoreCollisions in cups-progs, but I think
it's better if we use ignoreCollisions only if there are _real_
collisions between files with different contents.

Of course, we also check whether the file permissions match, so you get
a collision if contents are the same but the permissions are different.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>