CMake : add executable with unknown genererated source files - cmake

Question

I have a tool that generates a set source files whose name I am not able to know beforehand.
How to write a proper CMakeLists.txt script for scenario? This question has been asked before here CMake Compiling Generated Files. But it does not have a proper solution.
For instance, in the first answer (https://stackoverflow.com/a/8748478/2912478), OP can predict which files will generated based on the input .idl files. The second answer (https://stackoverflow.com/a/39258996/2912478) shows three different ways to solve but I really couldn't get his solution working.
Test case
I prepared a simple test case. Suppose I have this static file where the main resides (main.cpp).
# main.cpp
void foo(void);
int main() {
foo();
return 0;
}
Currently, I am using this CMakeLists.txt. The custom command generates the source file under src.
# CMakeLists.txt
add_executable(a.out main.cpp)
add_custom_command(
OUTPUT mylib.cpp
COMMAND ${CMAKE_SOURCE_DIR}/genf.sh ${CMAKE_SOURCE_DIR}/src
DEPENDS ${CMAKE_SOURCE_DIR}/genf.sh
)
add_custom_target(GenFile DEPENDS mylib.cpp)
add_dependencies(a.out GenFile)
Here is the hypothetical code generator genf.sh. I use a random number to mimic the fact that we do NOT know which files the generator will generate.
#!/bin/bash
rm -rf $1/*
fname=$(echo $((1 + RANDOM % 100)))
echo "Generating src$fname.cpp"
echo "void foo(void) {}" > $1/src$fname.cpp
Attempt 1
I tried to use GLOB to find all files generated. So I put the following lines at the end of my CMakeLists.txt. This doesn't work because at the moment of running cmake .. there is no files under src. So this solution never links the generated source files.
file(GLOB GeneratedSourceFiles ${CMAKE_SOURCE_DIR}/src/*.cpp)
target_sources(a.out PUBLIC ${GeneratedSourceFiles})