CMake, custom command and dependencies - cmake

Question

During my build process, a file generated by add_custom_command must the be compiled as C++. That's working but the generated file contains an include directive. When I modify the included file, the generated file is not recompiled.
There is an option IMPLICIT_DEPENDS for add_custom_command. But it does remake the generated file instead of just compiling it. And the documentation has a note
Note that the IMPLICIT_DEPENDS option is currently supported only for Makefile generators and will be ignored by other generators.
What's the recommended way to be able to recompile the generated file without remaking it and without depending on a given generator?
An example:
% more foo.h
#ifndef FOO_H
#define FOO_H
int const value = 36;
#endif
% more foo.in
#include "foo.h"
#include <iostream>
int main()
{
std::cout << "The answer is " << value << '\n';
return 0;
}
% more CMakeLists.txt
cmake_minimum_required(VERSION 3.15)
project(Test)
add_custom_command(OUTPUT foo.cpp
DEPENDS foo.in
COMMAND cp ${CMAKE_CURRENT_SOURCE_DIR}/foo.in foo.cpp)
set_source_files_properties(foo.cpp PROPERTIES INCLUDE_DIRECTORIES ${CMAKE_CURRENT_SOURCE_DIR})
add_executable(foo foo.cpp)
% mkdir build
% cd build
% cmake ..
...
% make
[ 33%] Generating foo.cpp
Scanning dependencies of target foo
[ 66%] Building CXX object CMakeFiles/foo.dir/foo.cpp.o
[100%] Linking CXX executable foo
[100%] Built target foo
% ./foo
The answer is 36
% sed -i.bak -e 's/36/42/' ../foo.h
% make
[100%] Built target foo
% ./foo
The answer is 36
I expect the second make to recompile foo.cpp (but not to regenerate it) so that the answer is 42.
Adding
IMPLICIT_DEPENDS CXX ${CMAKE_CURRENT_SOURCE_DIR}/foo.in
to the CMakeLists.txt generates a correct executable but regenerates foo.cpp when not needed and it not available for other generators according to the documentation.