Created attachment 384392 [details, diff]
patch against qtwidgets-5.3.1-r2.ebuild
Without xcb enabled at configure time, it does not compile in any support for leagcy x11 tray icons.
As a result, Qt5 does not report tray icon support on KDE4 or other x11 tray icon based systems.
Attached is a patch which makes it work again by adding the same useflag and configure options qtgui:5 uses.
I can confirm that the patch is enough to get at least
net-misc/owncloud-client-1.6.3[qt5 -qt4] started under
kde-4 with a working tray icon and without the message
"System tray not available
ownCloud requires on a working system tray. If you are running xfce, please follow these instructions. Otherwise, please install a system tray application such as trayer and try again."
Why [xcb=] and not [xcb?] ?
I didn't look at the configure system in depth, and keeping those settings the same accross both seemed the safer approach to me.
[xcb?] seems sufficient to me.
Fixed in git. Thanks for reporting and for the patch.