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

It seems Appveyor's images have been changed. The builds could not
find standard library headers anymore.

It seems to be an interaction between MS' scripts and the Windows
Driver Kit that is installed there.

One way to disable this interaction is to rename the `wdf` folder to
some other name.

See:
- http://stackoverflow.com/questions/31862627/vs2015-cl-cant-find-crt-libs-stido-h-ctype-h-etc-when-building-on-command-l
- http://help.appveyor.com/discussions/problems/3062-problem-with-new-updates-visual-studio-2015-and-cmake
- https://github.com/appveyor/ci/issues/414

Fix two context-free backend bugs