Fixed|qmake|OS X: Using the OpenGL framework in Snow Leopard

In the 10.6 build, OpenGL headers were accessed from the X11 include
directory even though the correct headers are in the OpenGL framework.
Fixed|GCC: Build failure casting templated functions

When applying a C style cast, gcc 4.2 loses track of some vital
"contextual information" and fails to build these casts.

de::function_cast<> seems to work around the issue.

(gcc 4.2 is being used in Snow Leopard.)