pure and const function attributes

[Marc Nieper-Wißkirchen]

Some GNULIB modules (e.g. xsize) use constructs like:

#if __GNUC__ >= 3
__attribute__ ((__pure__))
#endif

I am wondering whether it makes sense to add two new modules, named
pure and const that define macros GL_PURE and GL_CONST, respectively
that expand into nothing for a compiler that does not support these
function attributes and into the relevant code for a compiler that
does.

-- Marc