What is the minimal OpenGL version of the API required to compile VTK5.0?
The gl.h file provided by your compiler/system/SDK has to define at least the OpenGL 1.1 API. (Note: the functions and macros defined in higher OpenGL API versions or in other OpenGL extensions are provided by glext.h, glxext.h and wglext.h. Those 3 files are official files taken from http://opengl.org/registry/ and already part of the VTK source tree).