Don't generate/include config.h

Remove webvtt-config.h from libwebvtt(xx)_SOURCES
Always use strerror() in parsevtt demo

- No longer check for strerror() in configure.ac
- Conditional inclusion of stdint.h simplified
- config.h* removed from .gitignore
Merge pull request #389 from caitp/bug868629

Don't generate/include config.h