Support building with c2hs >= 0.18.2.

Includes limited support for c2hs 0.18.1, using a Cabal flag.
Fix compile warnings about void being imported from the wrong module.
Add support for COLOR_DEFAULT, used for translucent backgrounds.

Based on a patch by Simon Ruderich.
Bump version to 0.2.11.

Also fix a typo in the package description.