Ninja build

[tiennou]

A quick attempt at #4838. This also explicitly selects OpenSSL as a backend, just to make sure it doesn't auto-disable it.

[tiennou]

As suspected, the CMake version on 16.04 doesn't support Ninja, as per its confused error. Note that the only green Ubuntu build is because I missed switching it to Ninja in the first place.

It looks like it shaves almost a minute from the macOS build step, which is impressive, but ends up being slower overall (which looks to be caused by "generic network latency difference" between compared builds because of "differing datacenter usage" or something). It looks like grabbing Ninja from Homebrew also adds some delay (±15s when comparing "now" baselines).

Example list of builds :

• failed Ninja : https://dev.azure.com/libgit2/libgit2/_build/results?buildId=735&view=logs
• current Ninja : https://dev.azure.com/libgit2/libgit2/_build/results?buildId=736&view=logs
• current baseline (this morning) : https://dev.azure.com/libgit2/libgit2/_build/results?buildId=734&view=logs
• current baseline (now, failed) : https://dev.azure.com/libgit2/libgit2/_build/results?buildId=737&view=logs
• another baseline (later) : https://dev.azure.com/libgit2/libgit2/_build/results?buildId=739&view=logs

(Sorry for the notification noise, I was looking for some candidates PR for baselining)

[pks-t]

Cool, thanks for giving this a go! So from the results it looks like we might want to enable it for macOS, right? If it saves about a minute for build but requires 15 seconds more for homebrew that should be a net win of about 45 seconds

[tiennou]

Fixed and rebased !