configure.ac: Make our configure more consistent

This commit does the following changes:
 - Disable rpath by default
 - Switch all of our options to --enable-FEATURE in the help
 - Add auto-detection of libcap availability
 - Add auto-detection of python3 availability
 - Always specify the default value in --help
 - Add a configuration overview at the end

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>