GLEW 1.5.6 adds OpenGL 4.1 support

[Stefan]

GLEW 1.5.6 adds support for OpenGL 4.1, fixes bugs

*  New features:
          o Support for OpenGL 4.1
* New extensions:
          o GL_ARB_ES2_compatibility
          o GL_ARB_cl_event
          o GL_ARB_debug_output
          o GL_ARB_get_program_binary
          o GL_ARB_robustness
          o GL_ARB_separate_shader_objects
          o GL_ARB_shader_precision
          o GL_ARB_shader_stencil_export
          o GL_ARB_vertex_attrib_64bit
          o GL_ARB_viewport_array
          o GLX_ARB_create_context_robustness
          o GLX_EXT_create_context_es2_profile
          o WGL_ARB_create_context_robustness
          o WGL_EXT_create_context_es2_profile