Hi Harpinder,
Is there a reason you need to build from source? CentOS 7 is a supported platform for binary packages: Install MongoDB Community Edition on Red Hat or CentOS. I would recommend installing the latest version of the 3.6 production release series (currently 3.6.17) so you have the latest bug fixes and stability improvements. Minor releases do not include any backward-breaking changes, and there have been many improvements since MongoDB 3.6.9 was released in Nov, 2018.
If you definitely want to build from source, the v3.6.9-dbaas-testing branch isn’t what you are looking for (it’s a testing branch). Releases are tagged, so you want to use the v3.6.9 tag: Release r3.6.9 · mongodb/mongo · GitHub.
Regards,
Stennie