Package glew: Enable shared build & use original makefile.

- Using the upstream makefile looks like the cleaner approch.
- Still needs minor patching of the pkg-config files.
- For shared builds, we use -O0 instead of -O2, otherwise the resulting
  DLL does not work...
Merge pull request #350 from TobiX/glew-original-makefile

Package glew: Enable shared build & use original makefile.