ExternalProject_Add: how to use build dependencies in your project?

Question

I am trying to build a 3rd-party dependency using ExternalProject_Add (in this case libpqxx):

ExternalProject_Add(libpqxx_ext
    GIT_REPOSITORY
        https://github.com/jtv/libpqxx
    GIT_TAG
        tags/7.4.1
    GIT_SHALLOW
        true
    PREFIX
        ext
    BINARY_DIR
        build
    CMAKE_ARGS
        -DSKIP_BUILD_TEST=on
    INSTALL_COMMAND
        ""
    BUILD_ALWAYS
        OFF
    )

Once this step completes, the libpqxx artifacts I then need to use in my project are located at:

• header files: ${CMAKE_CURRENT_BINARY_DIR}/ext/src/libpqxx_ext/include
• static library: ${CMAKE_CURRENT_BINARY_DIR}/build/src/libpqxx.a

Use the results in my project:

How do I make these artifacts available to my project?

I have tried the following:

set(LIB_PQXX_SRC_DIR   ${CMAKE_CURRENT_BINARY_DIR}/ext/src/libpqxx_ext)
set(LIB_PQXX_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR}/build/src)

add_library(libpqxx STATIC IMPORTED GLOBAL)
set_property(TARGET libpqxx APPEND PROPERTY IMPORTED_LOCATION             ${LIB_PQXX_BIN_DIR}/libpqxx.a)
set_property(TARGET libpqxx APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${LIB_PQXX_SRC_DIR}/include)
set_property(TARGET libpqxx APPEND PROPERTY INTERFACE_LINK_LIBRARIES      ${PostgreSQL_LIBRARIES})

add_dependencies(libpqxx libpqxx_ext)

This doesn't work because at configure time the ${LIB_PQXX_SRC_DIR}/include doesn't exist yet.

CMake Error in foo/CMakeLists.txt: Imported target "libpqxx" includes non-existent path

"/home/steve/src/foo/build/3rd-party/pqxx/ext/src/libpqxx_ext/include"

in its INTERFACE_INCLUDE_DIRECTORIES. Possible reasons include:

• The path was deleted, renamed, or moved to another location.

• An install or uninstall procedure did not complete successfully.

• The installation package was faulty and references files it does not provide.

Adding add_dependencies to tell cmake that libpqxx_ext needs to be built before libpqxx doesn't work.

Note that if I remove libpqxx altogether, and then build my project (so that libpqxx_ext is built), and then add it, now everything works, because the ExternalProject_Add step has run... but obviously this breaks when I do a clean checkout, so it's not viable.

I also tried to tell cmake that the include dir is generated, but that had no effect

set_source_files_properties(${LIB_PQXX_SRC_DIR}/include PROPERTIES GENERATED TRUE)

Questions:

• Is my use of add_library(libpqxx STATIC IMPORTED GLOBAL) to import the artifacts the correct / recommended way of using the result of ExternalProject_Add?

• If it is, how can I tell cmake that ${LIB_PQXX_SRC_DIR}/include is a generated directory?

• If it is not, what is the recommended way of making the result of ExternalProject_Add available to my project?