Clean up build script for C++17/20

[GindaChen]

As mentioned in #99, we need to clean up the build script such that building in Ubuntu and macOS, with different language, can be easily done.

Major goals include:

1. Insulate brew dependency on macOS if the compiler is g++. Almost all tools in brew are built with clang, and if we need to use g++ we need to build most of them under the cmake toolchain. This is pretty doable.
2. Clean the install script and allow language version as an easy input. For example, ./build.sh --cpp20 should give us a full build on C++20. Suggestions are welcomed.

[GindaChen]

Moving to C++20 also implies we are by default using gcc-10. On (more modern) macOS, it should be fine - as long as we download gcc@10 through brew.

On Ubuntu 16, we will need to build gcc 10 manually - or at best, we host our own toolchain repo to download gcc 10. There is no apt-repo that Xenial can directly download gcc10 from.

If we switch to Ubuntu 18 / 20, then we're probably fine.

@pateljm @srsuryadev @mprammer2 I will be optimistic and try to move the dev stack and testing stack (CircleCI) to Ubuntu 18 20 if there is no objection.

[pateljm]

No objections at all - kudos instead!