Add incremental build

[dseefeld]

Add the following items to accomplish incremental builds in source-build:

• Add defined semaphore files for each target that participates
  • Semaphores in Directory.Build.props apply to top-level targets and projects, i.e. Directory.Build.targets, Build.proj, tools-local/init-build.proj, etc.
  • Semaphores in repos/Directory.Build.props apply to all repo level targets and projects.
• For each participating target, add Inputs (generally something larger and more static, like the project file) and assign the semaphore file as the output.
• For each participating target, create the semaphore file when the target completes.
• Add Timestamps to build binlogs to ensure that older binlogs don't get overwritten when build is restarted.

Accomplishes item #6 in #928 with the exception of clearing semaphore files for downstream repos when a repo starts building.

[dagood]

This doesn't look like it addresses a lot of #928, could you describe more what this PR is doing, what scenarios it's tested for?

[dagood]

Accomplishes item #6 in #928

It doesn't have the clearing part of it though, right?

When a repo starts building, all downstream repo semaphores should be removed so the new build's dependencies flow through to the end without question. This will help when making changes to e.g. CoreFX and expecting them show up in the Runtime without fully cleaning the repo first.

[dseefeld]

Accomplishes item #6 in #928

It doesn't have the clearing part of it though, right?

When a repo starts building, all downstream repo semaphores should be removed so the new build's dependencies flow through to the end without question. This will help when making changes to e.g. CoreFX and expecting them show up in the Runtime without fully cleaning the repo first.

Right. I missed that part. I'll update.

[dseefeld]

OSX issue is #1685