DerivedSources.make does not notice when FEATURE_DEFINES change
One possible solution would be for DerivedSources.make to write out some sort of FEATURE_DEFINES file in DerivedSources/ during a build, and then if the incoming FEATURE_DEFINES are ever different from that file, simply rebuild everything.
Sounds like a good idea to fix this.
I think this is critical to fix. I personally would fix it by eliminating FEATURE_DEFINES and using a file instead of an environment variable to communicate these settings. The build-webkit tool could generate the header file.
But I suppose the file would need to be a format that multiple tools could read, so perhaps it's impractical.
I wonder if this old bug is still valid. If it is, I wonder if landing bug 85456 will give us an easy way to fix it. :)