Fixed building with static Corrade.

Four tests (SceneGraph::Animable, Mesh, MeshTools::Tipsify,
TextureTools::Atlas) however fail, not sure what the cause is, as they
fail somewhere deep long after exiting from main().
Option to build Magnum as static library.

And optionaly position-independent code. The tests now pass again, if
both Corrade and Magnum is static.
Updated FindCorrade.cmake from Corrade repository.
FindMagnum.cmake: use CORRADE_*_LIBRARIES, not *_LIBRARY.