Yes, it is set by CMake. Grepping through the CMake code reveals, that for a host of compilers it is set. Probably they set it only for these compilers, which accepts this flag. Here one of the lines concerning GCC:
Modules/Compiler/GNU.cmake: set(CMAKE_${lang}_FLAGS_RELEASE_INIT "-O3 -DNDEBUG")
But be aware that many projects overwrite release/debug flags without preserving the initial setting and also overwriting user’s definitions.