CMAKE_CXX_FLAGS=-std=XXX does not override cxx_std_XX compile feature

I have a question regarding @brad.king comment on #16468 (comment 299093):

You can specify -std=XXX in CMAKE_CXX_FLAGS and it will be used everywhere. Do so prior to the project or enable_language command that enables the CXX language and CMake will be aware that the compiler's "default" mode is that and won't need to add other -std= flags to meet the feature requirements.

I do not observe the behavior described by Brad Kind above. If I try to set the std= flag on the command line, i.e. prior to project or enable_language, it does not look like CMake detects this as a default mode. Instead my code is compile with both -std=c++14 and -std=gnu++11.

$ cmake --version
cmake version 3.12.4

CMake suite maintained and supported by Kitware (kitware.com/cmake).
$ mkdir build
$ cd build
$ cmake -G Ninja -DCMAKE_CXX_FLAGS='-std=c++14' ..
-- The C compiler identification is GNU 8.2.1
-- The CXX compiler identification is GNU 8.2.1
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: .../cmake-cxx-standard/build
$ ninja -v
[1/2] /usr/bin/c++    -std=c++14   -std=gnu++11 -MD -MT CMakeFiles/hello.dir/hello.cpp.o -MF CMakeFiles/hello.dir/hello.cpp.o.d -o CMakeFiles/hello.dir/hello.cpp.o -c ../hello.cpp
[2/2] : && /usr/bin/c++  -std=c++14   CMakeFiles/hello.dir/hello.cpp.o  -o hello   && :

The test project available for download here: https://github.com/Sarcasm/cmake-cxx-standard.git

But the content can be summarized as:

$ tail -n +1 *
==> CMakeLists.txt <==
cmake_minimum_required(VERSION 3.12)

add_executable(hello hello.cpp)
target_compile_features(hello PRIVATE cxx_std_11)

==> hello.cpp <==
#include <iostream>

int main() {
  std::cout << "Hello world!\n";
  return 0;
}

Is it possible to override the C++ standard from the command line by using CXXFLAGS or CMAKE_CXX_FLAGS? I'm aware of CMAKE_CXX_STANDARD, but I inherit the CXXFLAGS from somewhere else.

Alternatively, is there a documented way to disable CMake own logic for the C++ standard? A year ago, another comment from Brad #17146 (comment 300071) indicated that there is a hack possible using CMAKE_CXX_STANDARD_DEFAULT, but no officially supported way yet.

I'm asking this for packaging related reasons, where many packages are built using the same CXXFLAGS and where a unified C++ ABI is targeted (all projects using the same C++ standard setting).