Bob opportunistically links against Qt4

Created by: anjos

The current way the build is structured, there is no flag that allows us switching off OpenCV and Qt4 dependences. If they are available on the target system, they are compiled and linked into the final binaries.

This behavior is incompatible with packaging tools in which you can describe to which packages another package may depend on. If Bob opportunistically links against what is available, the packaging system will not detect real dependences and avoid, for example, OpenCV to be removed without Bob being removed first.

For the time being, a solution would be to make the build systems always install all dependencies.

This bug was triggered by a bug reported to us on MacPorts: https://trac.macports.org/ticket/36491