Export GLLoadGen symbols so they can be used in shared libraries.

Caused linker problems mainly on Windows, where every symbol would need
to be exported manually, on Linux this was done implicitly (or with
`-fvisibility=default`). I now maintain glLoadGen fork at
https://github.com/mosra/glloadgen.git containing changes needed for
Magnum.

The `ogl_*()` functions are used only internally, various global
variables for extension queries are not used at all and thus they don't
need to be exported. I thus enabled `-fvisibility=hidden`.

Fixes #16.