Merge pull request #60 from htgoebel/master

Do not override existing user-config files.
Honour PREFIX environment variable if set. Requires GNU make.
Create 'uninstall' aliases for 'clean' targets.
Merge pull request #63 from JustAnotherGuy/master

Makefile changes
Revert "Honour PREFIX environment variable if set. Requires GNU make."

Better fix the documentation to show the correct usage.
This reverts commit 353587d1fab9fc67e0ff694d33f3d5d389c3ec91.