"Unity" (aka "jumbo", aka "amalgamation" builds concatenate source files
before compiling. It has these benefits for example: faster builds,
improved code optimization, cleaner code. Let's support and test this.
- enable unity builds for some existing CI builds to test this build
scenario.
- tune `UNITY_BUILD_BATCH_SIZE` size.
- disable unity build for example and test programs (they use one source
each already).
You can enable it by passing `-DCMAKE_UNITY_BUILD=ON` to cmake.
Supported by CMake 3.16 and newer.
Ref: https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.htmlCloses#1034