build glxconvenience also without Xrender

it contains #ifdefs for that, no need to disable it completely.

Change-Id: I4a7d03e09fefded966e2c3fec58a470b4f1d3300
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
the condition for building the x11 offscreen plugin is ... having xlib

Task-number: QTBUG-33240
Change-Id: Idc96239b0bcbe98d1519c239600aebcda42e8818
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>