util: Rename PURE to ATTRIBUTE_PURE.
util: Rename PURE to ATTRIBUTE_PURE.

To avoid collission with windows.h's PURE macro.

We could consider eventually renaming to __pure, but that would require
further care, so it's left to the future.

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler