add initial support for building with cmake

what works:
 - building QGLViewer
 - building examples
 - disabling VRender support
 - disable building examples
 - tested on archlinux using qt5 (5.2)

whats missing:
 - building the designer plugin
 - qt4/qt3 support
 - building contribs
 - adding translations
 - static builds
 - mac bundling
 - windows testing
 - ...?