Mongodb devel 2.7.7

Closes #32886.

The level of patching here is getting a little silly. Mongo are currently shifting around a fair bit of their codebase, and that is currently causing the HEAD build to fail.

The devel build also needs patching, discussed [here](https://jira.mongodb.org/browse/SERVER-15555), but that fix has been merged into the HEAD and consequently is scoped to only the devel build.

The last patch is the SConstruct file to accept 10.10 as a valid build option on Yosemite, which is in neither stable, devel or HEAD currently.
They've changed download links on everything. Not sure why, but this is the download link you now get if you meander to the website and hit the download link.