Make G_FUZZING constexpr, require -DBUILD_FOR_FUZZING=ON to execute a fuzz target

CMakeLists.txt

@@ -234,6 +234,10 @@ if(BUILD_FOR_FUZZING)
   set(BUILD_GUI_TESTS OFF)
   set(BUILD_BENCH OFF)
   set(BUILD_FUZZ_BINARY ON)
+
+  target_compile_definitions(core_interface INTERFACE
+    FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+  )
 endif()
 
 include(ProcessConfigurations)