Regression when setting CMAKE_CXX_FLAGS_RELEASE before project() since 3.11

Depending on the version of cmake, cmake does not generate compile_commands.json with the same flags.

With the following CMakeLists.txt:

cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -DFOO")
project(foo)
add_executable(foo foo.cpp)

invoking cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DCMAKE_BUILD_TYPE=Release .. on cmake 3.9 or 3.10 generates compile_commands.json containing:

[
{
  "directory": "/home/foo/build-cmake-3.9.6",
  "command": "/usr/bin/c++    -O3 -DNDEBUG   -o CMakeFiles/foo.dir/foo.cpp.o -c /home/foo/foo.cpp",
  "file": "/home/foo/foo.cpp"
}
]

With cmake >= 3.11, it generates the following file:

[
{
  "directory": "/home/foo/build-cmake-3.11.4",
  "command": "/usr/bin/c++     -DFOO   -o CMakeFiles/foo.dir/foo.cpp.o -c /home/foo/foo.cpp",
  "file": "/home/foo/foo.cpp"
}
]

Even though we should not specify CMAKE_CXX_RELEASE_FLAGS directly, some old codebase do it that way. Maybe cmake should emit a warning for theses cases?