* moves dolphin-specific settings out of Base.props
* creates exports.props for externals, allowing to easily import
  individual Externals
* corrects some cruft that accumulated and probably contributed
  to msbuild overbuilding

Ninja puts way more effort into compiling targets in parallel, and
ignores dependenceis until link time.

So we need to jump though hoops to force ninja to compile
pch.cpp before any targets which depend on the PCH.

I'm not sure if the previous implementation ever worked.