build: hit stoploss building boost on windows.

Building on windows is a nightmare. cl.exe doesn't support constexpr, boost
won't build with mingw. I've lost weeks to this, and it's enough.

This commit is a wip that tries to get boost building with mingw. I'm only
committing because I'm going to rip out all of the boost build stuff after
I tag this.

Until further notice, boost is header-only.