Not fully happy about the way this one interacts with the various
platforms. Maybe the platform_config.h should be generated by the
SCsub instead of passing a define just to know where is the header.
This allows us not to have to hack our definitions in the upstream files,
making it easier to upgrade to newer versions in the future.
For the include paths to work, the headers are moved to a GL subfolder to
match their upstream location.
