add_definitions(-DCOMPILE_UNIT_TESTS) (cf. CMake’s doc) or modify one of the flag variables (CMAKE_CXX_FLAGS, or CMAKE_CXX_FLAGS_<configuration>) or set COMPILE_FLAGS variable on the target.
Also, identifiers that begin with an underscore followed by an uppercase letter are reserved for the implementation. Identifiers containing double underscore, too. So don’t use them.