Update OpenGl_glext.h to the version shipped in OCCT 6.7.0
OpenGl_GlCore12.hxx: #undef GL_VERSION_x_y in all cases.
Revert "Fix prototype of debugCallbackWrap"

This reverts commit 943e8d684d133aa6e2a1f7f67a28bbc7d64a491b:
    Fix prototype of debugCallbackWrap

    Declaration in src/OpenGl/OpenGl_glext.h is:

      typedef void (APIENTRY *GLDEBUGPROCARB)(GLenum source,GLenum type,GLuint id,GLenum severity,GLsizei length,const GLchar *message,GLvoid *userParam);

    OCCT 6.7.0 ships an updated version of glext.h, which has a different
    signature (with respect to the last argument).  So maybe this change
    will be reverted if glext.h is updated.
Reapply commit 9721ffd against the new OpenGl_glext.h

  Remove GLcharARB and GLhandleARB definitions
  They are unused, and cause trouble on OSX Mavericks.
Reapply commit 6375238 against the new OpenGl_glext.h

  GL extensions under hpux

  Under hpux some opengl extensions are predefined by hp opengl
  having different ID'S.
Reapply commit cca4bae against the new OpenGl_glext.h

  Fix conflicting definitions of GLintptr on Mac OSX 10.5

  See discussion in issue #378.