GL_ARB_viewport_array is an extension mandated by OpenGL 4.1 as a means of exposing multiple viewports. Its specification is described at OpenGL.org.
Today's enabling of this OpenGL 4.1 feature is done just for the Intel Mesa DRI driver, aside from the core Mesa changes. This Intel support is also limited to Ivy Bridge graphics processors and newer.
The enabling of GL_ARB_viewport_array can be found as of this Git commit.