CMake FetchContent ignore build system

CMake FetchContent and ExternalProject bring remote projects into the top-level project to avoid making a monorepo or vendoring code into the top-level project. With FetchContent, the source code is retrieved at CMake configure time, allowing one to ignore the subproject build system and / or use only specific source files.

An example of this is using nRF5 SDK, which is a large project, but one may only wish to use a single source file and header as in this example: