win: use consistent RuntimeLibrary - inherit from node

This fixes a mismatch I noticed in how libgdal is built vs the node bindings. In windows land things can go pear shaped when this happens. Because node's build system sets `/MT` removing both of these options will result in the entire build consistently using `/MT`.

Appveyor is backed up, but will plan to merge this once https://ci.appveyor.com/project/brianreavis/node-gdal/build/418 is green.