[jcranmer]

There's two pieces to the exception.

The first piece is standard for compilers, and essentially says "our compiler linking in bits of this code into your code doesn't count." It means the license matters if you're modifying the compiler and support libraries yourself, but not if you're merely using the compiler.

The second piece revolves around the concerns of mixing licenses: Apache 2 and *GPLv3 are happy in combination, but that's because of some explicit compatibility text in GPLv3; this extends the compatibility text to GPLv2.