Fix build error simplifying logic used to include external projects

Since calling "find_package(Slicer [...])" already include ITK, it 
should be done conditionally in case the project is not build as a 
Slicer extension.

This should fix the following issue: http://slicer-devel.65872.n3.nabble.com/Extension-argument-list-length-error-td4028691.html

Note also that this commit does not address the other build error related to IO_WriteOnly and IO_Translate. QFile::WriteOnly should probably be used.