v8: give sensible error message if v8 sources are not checked out

v8: give sensible error message if v8 sources are not checked out

A common error for developers attempting to build qtbase for the first
time is to miss cloning the v8 submodule.

Let qmake check for existence of the sources so we get a sensible error
at qmake time, rather than a relatively inscrutable error at make time.

Change-Id: I70b478e63c962263dac4f2ddccb377b4c9777ceb
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>