The GLSL specification is vague here, (just says "as is standard for
C++"), though the C specifications seem quite clear that this should
be an error.
However, an existing piglit test (CorrectPreprocess11.frag) expects
this to be a warning, not an error, so we change this, and document in
README the deviation from the specification.
The README file had grown a little bit stale. We've been using newer
versions of both the GLSL and C99 specifications, so list those. Also,
several of the documented known limitations have since been fixed, so
remove those.
