Build external library only once with CMake

Question

My C++ projects includes the source code of a third-party library (currently as a git submodule).

This library is added to the project by our main CMakelists through

Answer 1

As @Tsyvarev said, in your case ExternalProject_Add is better than add_subdirectory. add_subdirectory is good when you want project to be essential part of your build system because target it creates can be used in in the right-hand-side of the target_link_libraries() command while target created by ExternalProject_Add cannot.

Here is the approach I used in one of my projects. You try to find required library and build it only if it was not found. I use INTERFACE library to turn FOO_EXTERNAL into a target acceptable by target_link_libraries().

add_library(foo INTERFACE)
find_package(foo ${FOO_VER})
if(NOT foo_FOUND)
    include(ExternalProject)
    include(GNUInstallDirs)
    ExternalProject_Add(FOO_EXTERNAL
                    SOURCE_DIR "${FOO_SOURCE_DIR}"
                    BINARY_DIR "${FOO_BINARY_DIR}"
                    INSTALL_DIR "${FOO_INSTALL_DIR}"
                    CMAKE_ARGS "-DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}"
                               "-DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}"
                               "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"

                               "-DCMAKE_INSTALL_PREFIX=${FOO_INSTALL_DIR}"
                    )

    add_dependencies(foo FOO_EXTERNAL)
    set(foo_LIBRARY
            "${FOO_INSTALL_DIR}/${CMAKE_INSTALL_LIBDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}foo${CMAKE_STATIC_LIBRARY_SUFFIX}")
    set(foo_INCLUDE_DIR "${FOO_INSTALL_DIR}/include")
endif()

target_link_libraries(foo INTERFACE ${foo_LIBRARY})
target_include_directories(foo INTERFACE ${foo_INCLUDE_DIR})