CMake offers a domain-specific language to describe how to configure and build a project. It is natural that variables describing particular conditions are introduced and conditional statements based on this are included in CMakeLists.txt.

In this recipe, we will revisit generator expressions, which we used throughout Chapter 4, Creating and Running Tests, to compactly refer to explicit test executable paths. Generator expressions offer a powerful and compact pattern for logical and informational expressions that are evaluated during build system generation and produce information specific to each build configuration. In other words, generator expressions are useful to refer to information that is only known at generation time, but not known or difficult to know at configure time; this is particularly the case for filenames, file locations, and library file suffixes.

In this example, we will employ generator expressions to conditionally set a preprocessor definition and conditionally link to a message passing interface (MPI) library and allow us to build the same source code either sequentially or using MPI parallelism.