I think @Dominik Kasewski's answer is more appropriate for what you want to do but there is also the configure_file mechanism

configure_file(myfile.h.in myfile.h)

CMake will take myfile.h.in and expand cmake variables in it:

static constexpr const char * build_type = "${CMAKE_BUILD_TYPE}";

to create a file that

static constexpr const char * build_type = "Debug";

for example.

• This is only useful if you have more than just one thing to do.
• If you have ${...} constructs in the file and dont want those expanded you can also use @CMAKE_BUILD_TYPE@ and add the @ONLY argument to configure_file
• This allows you to do more than just macros that you would set with target_compile_definitions.

Since this is more complicated than what @Dominik Kaszewski suggests, I would go with their answer but if you have more information that you want to pass from CMake to the source code, then this is a convenient way to do it.