gl_loadFunctions as needed by extension wrangler is not exported

[ghost]

When trying to compile the magnum-examples repo, the gl_loadFunctions symbol is never found. The symbol is included in libMagnum but not exported. Modifying CMakefiles to include the objects with GlutApplication allows examples to be built.

[mosra]

This one is hard. It seems that glLoadGen doesn't have any support for shared libraries. This is not a problem on Linux, where all symbols are exported by default, but on Windows every symbol needs to be marked explicitly. So I need to patch glLoadGen to export all the symbols.

The initial problem with ogl_LoadFunctions() is fixed in 100990c, but that covers just a small subset of the linker problems. Some inline functions (e.g. in Renderer.h) call gl*() functions directly and they will cause another linker errors when used.