Previous patches set up the piglit-dispatch infrastructure but did not
use it.  This patch enables piglit-dispatch and uses it instead of
GLEW.

- No piglit regressions on Intel SandyBridge (Linux) or the nVidia
  binary blob for Linux.

- One regression on Mac OSX: after this patch,
  shaders/gpu_shader4_attribs incorrectly skips.  This happens because
  the functions tested by shaders/gpu_shader4_attribs are defined in
  both EXT_gpu_shader4 and NV_vertex_program4 extensions.
  Piglit-dispatch is not yet capable of understanding that a function
  might be defined with the same name in multiple extension specs
  (because it is based on the gl.spec file from www.opengl.org, which
  fails to encode this information).  I plan to address this in a
  future patch.

- Spot checked on Microsoft Windows.

v2: Remove redundant includes of GL headers.