To make sure that the preprocessor variables are only defined if the CMake variable is considered a true constant, one can employ #cmakedefine instead of #define in the header files that are about to be configured, by using configure_file.
Depending on whether the CMake variable is defined and evaluates to a true constant, #cmakedefine YOUR_VARIABLE will then be replaced with either #define YOUR_VARIABLE ... or /* #undef YOUR_VARIABLE */. There is also #cmakedefine01, which will set a variable to either 0 or 1, depending on whether the variable is defined.
