[vcpkg] Optionally limit the number of cores during vcpkg bootstrap

[jherico]

Add an option to the bash bootstrap script that allows a user to force the use of only a single CPU during both compilation and linking of the vcpkg executable.

Building on low-memory / high-core count systems like 1-2 GB variants of Raspberry PIs will often fail because of memory exhaustion. Each compile will typically take a few hundred MB and running 4 of them concurrently will easily exhaust even a 2GB setup. Further, such systems often have no swap space configured. This command line option ensures that vcpkg will bootstrap successfully, albeit slowly, even when only 1GB of ram is available.

This PR does not attempt to address any similar issues that might arise from building packages using vcpkg

• What does your PR fix?

Fixes #14534

• Which triplets are supported/not supported? Have you updated the [CI baseline]

This is a modification to the bootstrap script and should have no impact on the triplets, only a potential impact on the speed of bootstrapping.

• Does your PR follow the maintainer guide?

Yes. The vast majority of the guide doesn't apply since this is a change to the boostrapping mechanism, not to any port or vcpkg functionality.

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

N/A

[aizuon]

Could this feature also be implemented when compiling packages? Would be nice to leave half of the cores unattended when compiling big packages, i.e. Qt, to do other work to spend time.

[jherico]

@aizuon It would be inconsistent. As far as I understand, not every package uses CMake for the build wrapper, and the approach here only works with CMake. It's likely that there may be some necessary similar feature that would be useful for package building, but this PR is just focused on the bootstrap process and unblocking people in the situation described above.

[vicroms]

@aizuon You can try setting the VCPKG_MAX_CONCURRENCY environment variable which most buildsystem helper functions support (there may be some ports with weird buildsystems that won't).

@jherico This LGTM. The team may have some opinions about the argument name, I'll bring it up in today's meeting.

[ras0219-msft]

I feel like it would make more sense to just make bootstrap respect VCPKG_MAX_CONCURRENCY

[jherico]

@ras0219 I've modified the build script to respect VCPKG_MAX_CONCURRENCY and removed the other modifications. I feel like it would be helpful to have some text in the -help output but I couldn't come up with something both informative and concise, so I left it as is.