AppVeyor: parallelize without running out of memory

[lightmare]

In short: run only 1 MSBUILD process, allow CL to use all processors, except on heavy sources where it's restricted to 1 (or 2 for heavier plugins)

It's possible to finish in 16 minutes: https://ci.appveyor.com/project/Mapbox/mapnik/history?branch=wip-msbuild-parallel

[wilhelmberg]

Great work, especially getting those heavy files more readable. I'll carry over some of your improvements, but I'm working on a version that requires less changes.

[lightmare]

The second commit (8f513b6 include common.gypi-appveyor conditionally) is completely optional, it's not needed for this to work.

As for the other two, the main criteria should be the level of parallelization, not the amount of changes. I don't run windows, so that's up to others to test, but here're some thoughts:

• we have heavy sources in:
  1. mapnik lib: /MP1 required on AppVeyor, and desirable outside AppVeyor as they need a lot of memory
  2. mapnik-json, mapnik-wkt: /MP1 required on AppVeyor, desirable outside
  3. plugins: csv, geojson, topojson: they don't need as much memory as the previous ones, but still more than most other sources, so I used /MP2 for them
• before this change MSBUILD is invoked 3 times:
  • compile heavy sources in mapnik lib
  • build heavy libs mapnik-json, mapnik-wkt
  • build mapnik lib and plugins
• that has some issues:
  • mapnik lib is the largest portion of the build, so you want to:
  • start building it as soon as possible
  • use all available processors for compilation /MP
  • outside AppVeyor with MSBUILD /m:2, mapnik lib won't start building until after both mapnik-json and mapnik-wkt are done
  • some plugins need more memory to compile, /MP must be limited for them on AppVeyor, which you can do by either
  • adding a fourth MSBUILD invocation, or
  • putting the information that a target is memory-intensive (must be compiled on fewer processors) directly on the target in mapnik.gyp

I added restrictive /MP1 or /MP2 flags on specific targets in mapnik.gyp (that's the first commit 2b8b57d detect AppVeyor inside mapnik.gyp, override /MP for hungry targets), and that allowed me to get rid of the second MSBUILD invocation. Now mapnik lib starts building immediately after the initial heavy sources compilation.