deskutils/owncloudclient: Add DEBUG option

Add a debug option allowing users to easily enable debug builds. This can help
isolation of particular bugs (like crashes) [1]

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203885

PR: 203919
Submitted by: Yonas Yanfa <yonas fizk net> (maintainer)