glcpp: Demote "macro redefined" from an error to a warning

glcpp: Demote "macro redefined" from an error to a warning

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.