compile each object file only once...

CMakeLists.txt

cmake_minimum_required(VERSION 3.5)
  
find_program(CCACHE_PROGRAM ccache)
if(CCACHE_PROGRAM)
    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE "${CCACHE_PROGRAM}")
endif()

set(program_name an_experiment)
project(${program_name} CXX)

set(VERSION "0.0.1")

function(set_common_properties_on_targets)
    foreach(target_name ${ARGN})
        message(STATUS "setting properties on: " ${target_name})
        set_target_properties(${target_name} PROPERTIES
            CXX_STANDARD 14
            CXX_STANDARD_REQUIRED YES
            CXX_EXTENSIONS NO
            ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib"
            LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib"
            RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin")
        target_compile_options(${target_name}
            PUBLIC "-O2"
            PUBLIC "-ggdb"
            PUBLIC "-grecord-gcc-switches"
            PUBLIC "-Wall"
            PUBLIC "-Wextra"
            PUBLIC "-fdiagnostics-color=auto")
        target_include_directories(${target_name} PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
        target_include_directories(${target_name} PUBLIC ${CMAKE_BINARY_DIR})
    endforeach()
endfunction()

function(create_object_files)
    foreach(src_files ${ARGN})
        message(STATUS "adding an object file: " ${src_files})
        string(REGEX REPLACE "\.cc$" "_o" name ${src_files})
        string(REGEX REPLACE "\/" "_" name ${name})
        add_library(${name} OBJECT ${src_files})
        set_common_properties_on_targets(${name})
    endforeach()
endfunction()

function(create_static_library)
    list(GET ARGN 0 name)
    list(REMOVE_AT ARGN 0)
    set(add_library_arguments "")
    foreach(src_file ${ARGN})
        string(REGEX REPLACE "\.cc$" "_o" object_file_name ${src_file})
        string(REGEX REPLACE "\/" "_" object_file_name ${object_file_name})
        list(APPEND add_library_arguments "$<TARGET_OBJECTS:${object_file_name}>")
    endforeach()
    add_library(${name} STATIC ${add_library_arguments})
    set_common_properties_on_targets(${name})
endfunction()

function(create_executable)
    list(GET ARGN 0 name)
    list(REMOVE_AT ARGN 0)
    set(add_executable_arguments "")
    foreach(src_file ${ARGN})
        string(REGEX REPLACE "\.cc$" "_o" object_file_name ${src_file})
        string(REGEX REPLACE "\/" "_" object_file_name ${object_file_name})
        list(APPEND add_executable_arguments "$<TARGET_OBJECTS:${object_file_name}>")
    endforeach()
    add_executable(${name} ${add_executable_arguments})
    set_common_properties_on_targets(${name})
endfunction()

create_object_files(
    src/foo.cc
    src/bar.cc
    src/baz.cc
    src/main.cc)

create_static_library(foobarbaz
    src/foo.cc
    src/bar.cc
    src/baz.cc)

create_static_library(foobar
    src/foo.cc
    src/bar.cc)

create_static_library(foobaz
    src/foo.cc
    src/baz.cc)

create_static_library(barbaz
    src/bar.cc
    src/baz.cc)

create_static_library(baz
    src/baz.cc)

create_static_library(bar
    src/bar.cc)

create_static_library(foo
    src/foo.cc)

create_executable(main
    src/main.cc)
target_link_libraries(main
    barbaz
    foo)
target_include_directories(main PUBLIC src)