Various build system improvements.

 * Try to use -Bsymbolic to avoid PLT jumps.
 * Detect Windows (define OS_LINUX OS_WINDOWS)
 * Verify basic ABI compliance with abicheck.sh and libbson.symbols.
 * Cleanup bson/Makefile.include.

Libbson is pretty aggressive about warning the consumer of invalid API
use. However, that means that we short circuit some functions. This is
valid since consumers shipping invalid API use is nothing we can protect
against, but this will at least quiet some static analyzers.

Add blurb on fast mutation.