CMake using conditionals

CMakeLists.txt

cmake_minimum_required(VERSION 3.17)
project(cmake_recipe01 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 14)

set(USE_LIBRARY ON)
message(STATUS "Compile sources into library? ${USE_LIBRARY}")

set(BUILD_SHARED_LIBS OFF)
list(APPEND _sources Message.h Message.cpp)

if(USE_LIBRARY)
    add_library(message ${_sources})
    add_executable(cmake_recipe01 main.cpp)
    target_link_libraries(cmake_recipe01 message)
else()
    add_executable(cmake_recipe01 main.cpp ${_sources})
endif()