Promote target to thirdparty

examples.cmake

external_dependency(rttr https://github.com/rttrorg/rttr.git v0.9.6)                                                                    17 external_dependency(MetaStuff https://github.com/Manu343726/MetaStuff master)                                                        +  18 mark_as_external_target(rttr_core)                                                                                                   
mark_as_external_target(rttr)    

output.log

[47/112] Building CXX object examples/rttr-build/src/rttr/CMakeFiles/rttr_core.dir/destructor.cpp.o                                        
FAILED: examples/rttr-build/src/rttr/CMakeFiles/rttr_core.dir/destructor.cpp.o                                                             
/usr/bin/clang++  -DRTTR_DLL -DRTTR_DLL_EXPORTS -Drttr_core_EXPORTS -Iexamples/rttr-build/src/rttr/.. -Iexamples/rttr-src/src/rttr/.. -frtt
i -O3 -DNDEBUG -frtti -g -fPIC -fvisibility=hidden -fvisibility-inlines-hidden   -Werror -Wall -std=c++14 -MD -MT examples/rttr-build/src/r
ttr/CMakeFiles/rttr_core.dir/destructor.cpp.o -MF examples/rttr-build/src/rttr/CMakeFiles/rttr_core.dir/destructor.cpp.o.d -o examples/rttr
-build/src/rttr/CMakeFiles/rttr_core.dir/destructor.cpp.o -c examples/rttr-src/src/rttr/destructor.cpp
In file included from examples/rttr-src/src/rttr/destructor.cpp:29:
In file included from examples/rttr-src/src/rttr/../rttr/detail/destructor/destructor_wrapper_base.h:32:
In file included from examples/rttr-src/src/rttr/../rttr/type.h:1221: 
In file included from examples/rttr-src/src/rttr/../rttr/detail/type/type_impl.h:36:
In file included from examples/rttr-src/src/rttr/../rttr/detail/type/get_create_variant_func.h:32:
In file included from examples/rttr-src/src/rttr/../rttr/variant.h:1156:
In file included from examples/rttr-src/src/rttr/../rttr/detail/variant/variant_impl.h:36:
In file included from examples/rttr-src/src/rttr/../rttr/detail/variant/variant_data_policy.h:36:
In file included from examples/rttr-src/src/rttr/../rttr/detail/variant/variant_data_converter.h:32:
examples/rttr-src/src/rttr/../rttr/detail/conversion/number_conversion.h:137:16: error: implicit conversion from 'int' to 'float' changes v
alue from 2147483647 to 2147483648 [-Werror,-Wimplicit-int-float-conversion]
    if (from > std::numeric_limits<T>::max())
             ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

system_target.cmake

function(mark_as_external_target TARGET)                                                                                             + 470     get_property(type TARGET ${TARGET} PROPERTY TYPE)                                                                                + 471                                                                                                                                      
    if(NOT (type STREQUAL "INTERFACE_LIBRARY"))                                                                                      
        get_property(includes TARGET ${TARGET} PROPERTY INCLUDE_DIRECTORIES)                                                         + 474     endif()                                                                                                                          
    get_property(interface_includes TARGET ${TARGET} PROPERTY INTERFACE_INCLUDE_DIRECTORIES)                                         
                                                                                                                                      
    set_property(TARGET ${TARGET} APPEND PROPERTY INTERFACE_SYSTEM_INCLUDE_DIRECTORIES ${includes} ${interface_includes})            
endfunction()  

Try this instead. CMake should take care of it properly from here on out.

function(mark_as_external_target target)
  get_property(type TARGET ${target} PROPERTY TYPE)
  set(property INTERFACE_INCLUDE_DIRECTORIES)
  set(visibility PUBLIC)
  if (type STREQUAL "INTERFACE_LIBRARY")
    set(visibility INTERFACE)
  endif()
  get_property(includes TARGET ${target} PROPERTY ${property})
  target_include_directories(${target} ${visibility} SYSTEM ${includes})
  if (NOT (type STREQUAL "INTERFACE_LIBRARY"))
    get_property(includes TARGET ${target} PROPERTY INCLUDE_DIRECTORIES)
    target_include_directories(${target} PUBLIC SYSTEM ${includes})
  endif()
endfunction()