configure: Give a reason for disabled features

This adds a short reason to the summary at the end of configure (for options
where it makes sense, e.g. not debug or ipv6).

Example:

  charset:      no (icu-uc not found via pkg-config)

Signed-off-by: Uli Schlachter <psychon@znc.in>
Merge pull request #496 from psychon/config-reasoning

configure: Give a reason for disabled features