Merge zlib, curl and libxml2 into a single job

[Steelskin]

These 3 jobs are all configured the same way and do not need to be separate jobs each spinning a new build bot instance.

Fixes #825

[compnerd]

I'm not sure if we want to necessarily merge these into a single job, there is some value in the parallelism still. I was thinking more that we want to merge the Darwin and Windows builds into a single instance rather than a separate set for Darwin and a separate set for Windows.

[Steelskin]

I'm not sure if we want to necessarily merge these into a single job, there is some value in the parallelism still. I was thinking more that we want to merge the Darwin and Windows builds into a single instance rather than a separate set for Darwin and a separate set for Windows.

Ah you would like to re-merge the flows instead? That's something that should be doable after the mac work is finished. I think this is still a good change because this is only consumed by the sdk job, which is happening after the compilers job so we don't get much from parallelization and this should be a bit cheaper to run since we don't need to spawn new instances for every sub-job.

[compnerd]

I am unsure about the merge because I'm thinking that we may want to consume zlib and libxml2 in the compilers build as well, but not CURL.