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>