[CMake] CMake custom target doesn't build

[Gil Moses]

Hi,
I'm posting this after getting no useful reply in StackOverflow: https://stackoverflow.com/questions/50198141/cmake-custom-target-doesnt-build.

Using CMake 2.8.8, I'm building two targets using:
add_library(tgt1 SHARED a.cpp)
add_library(tgt2 SHARED b.cpp)
After both are built, I need to run a post build step that depends on both targets. I tried many combinations of the following but with no success:
add_custom_target(final_tgt DEPENDS tgt1 tgt2)
add_custom_command(TARGET final_tgt POST_BUILD COMMAND <command> ARGS <args>)
The final target would simply not build, even though its build.make contains the custom command.
Tried to use ALL for the custom target, however make attempts to build it first while missing the first targets.
And I can't use an add_library or add_executable for the final target, since they require specifying source files.
What is the correct way to do it?
===================================
Edit: below is a minimal verifiable source code. What it attempts to do is to compile code (for Mac) in two architectures and as a post-build to create a universal binary using lipo:
cmake_minimum_required(VERSION 2.8)
set(icpc_req_path "/usr/local/bin/icpc-16.0.146")

set(CMAKE_CXX_COMPILER "${icpc_req_path}")

project("CMakeTest")
set(SOURCE_FILES a.cpp)

set (TARGET_NAME "TGT")
set(TARGETS "")
set(ARCHITECTURES i386 x86_64)

foreach(ar ${ARCHITECTURES})
    set(CMAKE_CXX_FLAGS_RELEASE "")
    set(CMAKE_CXX_FLAGS_DEBUG "")
    set(CMAKE_CXX_FLAGS "")

    add_library(TGT_${ar} SHARED ${SOURCE_FILES})
    set_target_properties(${TARGET_NAME}_${ar} PROPERTIES COMPILE_FLAGS "-arch ${ar} -xSSE3")
    set_target_properties(${TARGET_NAME}_${ar} PROPERTIES LINK_FLAGS "-arch ${ar}")
    set(TARGETS "${TARGETS};lib${TARGET_NAME}_${ar}.dylib")
endforeach(ar)

message("Targets: ${TARGETS}")
add_custom_target(${TARGET_NAME} DEPENDS ${TARGETS})
add_custom_command(TARGET ${TARGET_NAME} POST_BUILD COMMAND "lipo" ARGS "-create" ${TARGETS} "-output" "${TARGET_NAME}.dylib")

And the contents of a.cpp is:
int main(){}

Thanks,
Gil.

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