Various Fixes, master branch (2019.02.26.)

Unfortunately there are multiple things wrong in the master branch at the moment. 😦

While I decided to merge in !422 (merged), as it was behaving correctly for me locally, the CI did get confused by it. Causing build failures in all current MRs. (!447 (merged), !446 (merged)) A clean build of SoQt should fix this, so I added code to the package that can be used to trigger a clean build.

@rbianchi, I also added (back) the forceconfigure step for SoQt. Since SoQt depends on Coin3D, unless we actively reconfigure it on each build, changes in Coin3D could go unnoticed by the SoQt build. It's annoying, but this is why we have such forceconfigure steps in a few places...

At the same time, while trying to work with the master branch on macOS I noticed that with !445 (merged) I created a duplicate module. (FindLibXslt.cmake vs. Findlibxslt.cmake.) Since the old/existing one is clearly the better one, I just removed the one that I added by mistake.

Finally, to silence CMP0072 on systems that have hardware accelerated versions of OpenGL installed, I told CMake to use the "legacy" OpenGL driver by default. (This was already its behaviour, I just silenced a warning about it.)