Disable Qbs support when compiling with Qt4

Enabling C++11 for compiling the Qbs library gets pretty complicated
with Qt4. Mixing is not advisable (at least not on OS X where a
different stdlib is used in that case).

Change-Id: Ibad56dcf41bfed61f4d8ec8eb46c980768f24faf
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Mac/Qt4: Allow compilation with macx-clang mkspec too

Change-Id: I91752859f790b5576db7ec8678c58924f643187c
Reviewed-by: Christian Stenger <christian.stenger@digia.com>