Do build the cmdmail library, the uri-encode executable and do install
the senddoc script for OS X, too.

Note that in order for it to work, one needs to set the "E-mail
program" in Preferences:Internet:E-mail to /Applications/Mail.app. (Or
possibly some other application and/or executable.)

Change-Id: I5764c9891865983d46081edc854e321643c296cc