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
configure: Add --enable/disable-charset

This override the auto detection. If --enable-charset is given, but icu can't be
found, configure will error out.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Merge pull request #498 from psychon/configure-flag-for-charset

configure: Add --enable/disable-charset
Fix chansaver's channel keys by adding other OnMode() hooks
configure: Fix --disable-charset

Somehow I ended up only testing the automatic detection and the error message
from --enable-charset. I totally forgot that --disable-charset should skip the
whole check.

Thanks to Robby for reporting this.

Signed-off-by: Uli Schlachter <psychon@znc.in>