configure.ac: remove enable 32/64 bit hacks

These two were added ages ago, with an explicit comment "Hacks ..."
They have been insufficient for years and maintainers needed to
explicitly handle the build themselves.

Rather than lying and pretending that it works, just kill this hack and
let maintainers build things the way it should be done for their
distribution.

Document the removal in the release notes.

Suggested-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

docs/relnotes/10.3.html

@@ -77,6 +77,8 @@ TBD.
 
 <ul>
 <li>Removed support for the GL_ATI_envmap_bumpmap extension</li>
+<li>The hacky --enable-32/64-bit is no longer available in configure. To build
+32/64 bit mesa refer to the default method recommended by your distribution</li>
 </ul>
 
 </div>