mesa: Add new functions and enums required by GL_ARB_sample_shading

mesa: Add new functions and enums required by GL_ARB_sample_shading

New functions added by GL_ARB_sample_shading:
glMinSampleShadingARB()

New enums:
GL_SAMPLE_SHADING_ARB
GL_MIN_SAMPLE_SHADING_VALUE_ARB

V2: Update comments.
    Create new GL4x.xml.
    Remove redundant code in get.c.
    Update the API_XML list in Makefile.am.
    Add extra_gl40_ARB_sample_shading predicate to get.c.

V3:
   Fix make check failure.
   Add checks for desktop GL.
   Use GLfloat in place of GLclampf in glMinSampleShading().
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ken Graunke <kenneth@whitecape.org>