What I’d do in this situation is create an INTERFACE target with all the flags, defines, features, etc. on it and then link to $<BUILD_INTERFACE:mybuildflags> from each target. I don’t think there’s any way to do this to every target other than asking CMake for each target and adding it at the top level. The only way to iterate looks to be iterating over the BUILDSYSTEM_TARGETS property of each directory you have (and filtering out non-compiling targets).
However, what you can do is at least set CMAKE_CXX_STANDARD to 14. That doesn’t guarantee that the compiler supports all 14’s features, but it should at least ask.