That makes it much easier to handle in that case (because that's the
whole point of static builds) and also nicely circumvents any issues
with duplicated global data when a static lib would be linked to
multiple dynamic libraries.
Some non-trivial tricks had to be done in order to expose the
GL::defaultFramebuffer variable without causing leaks or double frees.
Also, the AbstractFramebuffer has a protected destructor so this needs
another special handling.
Vladimír Vondruš