[Blue Sky] Overhaul build scripts

[wilhelmberg]

Master ticket to capture ideas to improve build scripts

• different overall approach
  • port to mason [Blue sky] port to Mason #25
  • use alternative utilities, e.g. msys: Alternative utilities #7
  • use PowerShell ScriptCs (see below) instead of Batch for most tasks Port scripts to powershell #24
• reactivate Debug builds x64 debug build does not work at all. #58
• support compilers other than VS/MSBuild
  • clang
  • MinGW [Feature request] Mapnik 3.0 SDK built with MinGW-w64 #50
• speed up builds: Speed up builds #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