[Blue Sky] Overhaul build scripts

[wilhelmberg]

Master ticket to capture ideas to improve build scripts

• different overall approach
  • port to mason https://github.com/mapbox/windows-builds/issues/25
  • use alternative utilities, e.g. msys: https://github.com/mapbox/windows-builds/issues/7
  • use ~~PowerShell~~ ScriptCs (see below) instead of Batch for most tasks ~~https://github.com/mapbox/windows-builds/issues/24~~
• reactivate Debug builds https://github.com/mapbox/windows-builds/issues/58
• support compilers other than VS/MSBuild
  • clang
  • MinGW https://github.com/mapbox/windows-builds/issues/50
• speed up builds: https://github.com/mapbox/windows-builds/issues/64
• have a common download directory for 32 and 64bit builds so that deps are downloaded only once https://github.com/mapbox/windows-build-server/issues/2#issuecomment-76900540

[wilhelmberg]

Not going to go he Powershell route.

Decided to go with ScriptCs: scripts/build.csx [wip] for mapink: https://github.com/mapnik/mapnik-gyp/compare/new-win-build-scripts