get_property could not find TARGET, from ExternalProject

Question

I'm running into a CMake issue that I am a little stumped by. It involves a CMake project that builds dependencies for an application I develop at work. I now have to add a new dependency, libnest2d, which itself also has three dependencies, of which some are also new.

To get libnest2d to build I have this ExternalProject_Add call:

ExternalProject_Add(libnest2d
    GIT_REPOSITORY https://github.com/tamasmeszaros/libnest2d.git
    GIT_TAG da4782500da4eb8cb6e38e5e3f10164ec5a59778
    CMAKE_ARGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
               -DCMAKE_PREFIX_PATH=${CMAKE_INSTALL_PREFIX}
               -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
    DEPENDS BoostHeaders nlopt Clipper
)

This project depends on the Boost Headers, NLopt and Clipper. Those Boost headers and Clipper are fine, but it somehow complains about NLopt with the following errors:

CMake Error at /usr/share/cmake-3.16/Modules/ExternalProject.cmake:2962 (get_property):
  get_property could not find TARGET nlopt.  Perhaps it has not yet been
  created.
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:3239 (_ep_add_configure_command)
  projects/libnest2d.cmake:5 (ExternalProject_Add)
  CMakeLists.txt:61 (include)


CMake Error at /usr/share/cmake-3.16/Modules/ExternalProject.cmake:2964 (get_property):
  get_property could not find TARGET nlopt.  Perhaps it has not yet been
  created.
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:3239 (_ep_add_configure_command)
  projects/libnest2d.cmake:5 (ExternalProject_Add)
  CMakeLists.txt:61 (include)


CMake Error at /usr/share/cmake-3.16/Modules/ExternalProject.cmake:1783 (get_property):
  get_property could not find TARGET nlopt.  Perhaps it has not yet been
  created.
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:2064 (ExternalProject_Get_Property)
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:2966 (_ep_get_step_stampfile)
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:3239 (_ep_add_configure_command)
  projects/libnest2d.cmake:5 (ExternalProject_Add)
  CMakeLists.txt:61 (include)


CMake Error at /usr/share/cmake-3.16/Modules/ExternalProject.cmake:1785 (message):
  External project "nlopt" has no stamp_dir
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:2064 (ExternalProject_Get_Property)
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:2966 (_ep_get_step_stampfile)
  /usr/share/cmake-3.16/Modules/ExternalProject.cmake:3239 (_ep_add_configure_command)
  projects/libnest2d.cmake:5 (ExternalProject_Add)
  CMakeLists.txt:61 (include)

So it states that the nlopt target is not defined. However that target is defined using another ExternalProject_Add call, the same as the other two dependencies:

ExternalProject_Add(nlopt
    GIT_REPOSITORY https://github.com/stevengj/nlopt.git
    GIT_TAG v2.6.2
    CMAKE_ARGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
               -DCMAKE_PREFIX_PATH=${CMAKE_INSTALL_PREFIX}
               -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
)

ExternalProject_Add(BoostHeaders
    URL http://sourceforge.net/projects/boost/files/boost/1.67.0/boost_1_67_0.tar.bz2
    URL_HASH SHA1=694ae3f4f899d1a80eb7a3b31b33be73c423c1ae
    CONFIGURE_COMMAND ""
    BUILD_COMMAND ""
    INSTALL_COMMAND "${CMAKE_COMMAND}" -E copy_directory "${CMAKE_CURRENT_BINARY_DIR}/BoostHeaders-prefix/src/BoostHeaders/boost" "${CMAKE_INSTALL_PREFIX}/include/boost"
)

ExternalProject_Add(Clipper
    URL https://sourceforge.net/projects/polyclipping/files/clipper_ver6.4.2.zip
    URL_HASH SHA1=b05c1f454c22576f867fc633b11337d053e9ea33
    SOURCE_SUBDIR cpp
    CMAKE_ARGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
               -DCMAKE_PREFIX_PATH=${CMAKE_INSTALL_PREFIX}
               -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
)

Without the NLopt dependency, libnest2d builds fine (since I still have the dependency installed on my local system), but there is no guarantee that NLopt is built and installed before starting on libnest2d, and indeed this goes wrong if I execute this build in a VM with multiple threads.

If I temporarily remove the nlopt dependency and call cmake .. && make help then I see that nlopt is one of the available targets. I can also call make nlopt and it starts building NLopt as expected.

You can view my entire source code here: https://github.com/Ultimaker/cura-build-environment/tree/CURA-7259_pynest2d . As of this writing I'm on commit 39298d203d115b60d7093f0a801be1bad0ba7842.

Other problems I've found have not been the same and don't provide a solution for me:

• This related question has the same error but it seems to be caused by a tool that OP was using, which I'm not using.
• This question and this bug report were a problem with a target that was disabled by a build option, which is not the case for me. The target clearly exists and I made no option to disable it.
• There was an old bug that caused this but it was fixed in CMake 2.8. I'm using CMake 3.16.3.

So to summarize, how can I cause the libnest2d external project to depend on NLopt? Why are two dependencies accepted, but one isn't?