LLVM needs _GNU_SOURCE defined, so let the SConscript handle that for the regex backend and BSD compat.

Allocate 256 bits, not 256 bytes, for a charset

Fix charset conflicts

Report correct token position from LR backends

Iterative (chunked) input processing

Remove now unneeded workaround

The workaround would now cause build failures in Appveyor as seem to
have pre-emptively applied it to their environments.

I'm not too happy about their handling of the issue. Seems fiddly.

Anyway this commit is now required to build on Appveyor.

Add workaround for VS2015 so that appveyor works