build: make build faster (at least on Windows) #12281

• Version: *
• Platform: Windows
• Subsystem: build

The problem: even for minor changes building on Windows can take 10s of minutes.
This issue is for tracking several approaches to mitigate that.

• make GYP less aggressive in recreating *.*proj files
• better signaling to VS of the minimal change set
• use compiler cache
• make using ninja simpler
• allow for debug build to use /lib/ files as is (skip mksnapshot)
• shortcut the ICU toolchain
• allow decoupling of /deps/ from main executable compilation & linking
• easy way to cloud-source parts of the compilation