This removes a custom redefinition of glewGetProcAddress,
which was apparently necessary for older Macs with buggy OpenGL
(see #5087).
The added source files are as provided in the upstream tarball.
This allows us not to have to hack our definitions in the upstream files,
making it easier to upgrade to newer versions in the future.
For the include paths to work, the headers are moved to a GL subfolder to
match their upstream location.
