Fix compile dependencies of examples.

Touching an external header wouldn't trigger the recompilation
of example source files since the Makefile didn't list them as
dependencies. The problem was that qmake didn't have its
dependency directories set properly since our default_pre.prf
would totally override the default one.

Change-Id: I1cbb6b259c15a70b872241cf257a6b5f93244cb4
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>