add_custom_command fails when OUTPUT is generated into a subdirectory

Code
3

VERSION 1 already automatically creates the subfolder generate as expected. Still, I added the following code just before the existing add_custom_command part:

# Create the directory before trying to use it!
if(EXISTS OUTPUT_DIRECTORY)
	message("Creating output directory '${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_DIRECTORY}'")
	file(MAKE_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_DIRECTORY}")
endif()

# Add command to generate source code files before building the target.
add_custom_command(
...

The configuration for VERSION 1 works fine, but building throws the error:

x86_64-w64-mingw32-g++.exe: error: <CMAKE_BINARY_DIR>\my-lib\test-source-1.cpp: No such file or directory

which is correct, since the path should be <CMAKE_BINARY_DIR>\my-lib\generate\test-source-1.cpp.

The configuration for VERSION 2 does not work, it throws the error:

[cmake] -- Configuring done
[cmake] CMake Error at my-app/CMakeLists.txt:8 (add_executable):
[cmake]   Cannot find source file:
[cmake] 
[cmake]     <CMAKE_SOURCES_DIR>/my-lib/generate/test-source-2.h
[cmake] 
[cmake]   Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
[cmake]   .hpp .hxx .in .txx
[cmake] 
[cmake] 
[cmake] -- Generating done

which of course does not work, since the file is generated into the <CMAKE_BINARY_DIR>.

So I added a third version specifying the full absolute path to the generated files:

...
elseif(${VERSION} EQUAL 3)
	# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	# This does NOT work.
	# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	set(OUTPUT_DIRECTORY "generate")

	set(GENERATED_HEADER "${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_DIRECTORY}/test-source-${VERSION}.h")
	set(GENERATED_IMPLEMENTATION "${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_DIRECTORY}/test-source-${VERSION}.cpp")

	set(INPUT_HEADER_FILE "${CMAKE_CURRENT_SOURCE_DIR}/test-source-${VERSION}.h.in")
	set(OUTPUT_HEADER_FILE "${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_DIRECTORY}/test-source-${VERSION}.h")

	set(INPUT_IMPLEMENTATION_FILE "${CMAKE_CURRENT_SOURCE_DIR}/test-source-${VERSION}.cpp.in")
	set(OUTPUT_IMPLEMENTATION_FILE "${CMAKE_CURRENT_BINARY_DIR}/${OUTPUT_DIRECTORY}/test-source-${VERSION}.cpp")
endif()

...

elseif(${VERSION} EQUAL 3)

	set(TARGET_SOURCES_HEADER_FILE "${GENERATED_HEADER}")
	set(TARGET_SOURCES_IMPLEMENTATION_FILE "${GENERATED_IMPLEMENTATION}")
endif()

message("INPUT_HEADER_FILE is: ${INPUT_HEADER_FILE}")
message("OUTPUT_HEADER_FILE is: ${OUTPUT_HEADER_FILE}")
...

set(TARGET_SOURCES_HEADER_FILE ${TARGET_SOURCES_HEADER_FILE} PARENT_SCOPE)
set(TARGET_SOURCES_IMPLEMENTATION_FILE ${TARGET_SOURCES_IMPLEMENTATION_FILE} PARENT_SCOPE)

and in the main CMakeLists.txt I am marking the source files as GENERATED as I have read somewhere in this forum:

...

add_subdirectory(my-lib)

message("Parent: TARGET_SOURCES_HEADER_FILE ${TARGET_SOURCES_HEADER_FILE}")
message("Parent: TARGET_SOURCES_IMPLEMENTATION_FILE ${TARGET_SOURCES_IMPLEMENTATION_FILE}")

set_source_files_properties(${TARGET_SOURCES_HEADER_FILE} GENERATED)
set_source_files_properties(${TARGET_SOURCES_IMPLEMENTATION_FILE} GENERATED)

# -------------------------------------------------------------------------------
# Executables
# -------------------------------------------------------------------------------

add_subdirectory(my-app)

Still this does not work.