The code for this recipe is available at https://github.com/dev-cafe/cmake-cookbook/tree/v1.0/chapter-01/recipe-05 and has a C++ example. The recipe is valid with CMake version 3.5 (and higher) and has been tested on GNU/Linux, macOS, and Windows.
In the previous recipe, we introduced conditionals in a rather rigid fashion: by introducing variables with a given truth value hardcoded. This can be useful sometimes, but it prevents users of your code from easily toggling these variables. Another disadvantage of the rigid approach is that the CMake code does not communicate to the reader that this is a value that is expected to be modified from outside. The recommended way to toggle behavior in the build system ...
