How can CMake arguments be forwarded to ExternalProject

Question

I have (mostly) successfully set up ExternalProject_Add for googletest. However, I noticed that things like my choice of C++ compiler, build type, etc. are not automatically

Answer 1

After long hours of trying to figure this out, it was finally answered (by Don Hinton) in the CMake mailing list. Fraser's solution is very close, but can still pass some project specific arguments that can cause some unpredictable behavior.

The following works properly. Hopefully this will save people some time trying to figure this out:

cmake_minimum_required(VERSION 3.1)

# MUST be done before call to 'project'
get_cmake_property(vars CACHE_VARIABLES)
foreach(var ${vars})
  get_property(currentHelpString CACHE "${var}" PROPERTY HELPSTRING)
    if("${currentHelpString}" MATCHES "No help, variable specified on the command line." OR "${currentHelpString}" STREQUAL "")
        # message("${var} = [${${var}}]  --  ${currentHelpString}") # uncomment to see the variables being processed
        list(APPEND CL_ARGS "-D${var}=${${var}}")
    endif()
endforeach()

project(SuperBuild)

include(ExternalProject)

ExternalProject_Add(ext_proj
  ...

  CMAKE_ARGS ${CL_ARGS}
)

Link to mailing list thread: https://cmake.org/pipermail/cmake/2018-January/067002.html