build: MP_UNITS_BUILD_AS_SYSTEM_HEADERS restored (but renamed)

src/CMakeLists.txt

@@ -36,9 +36,16 @@ include(CheckLibcxxInUse)
 check_libcxx_in_use(${projectPrefix}LIBCXX)
 
 # project build options
+option(${projectPrefix}BUILD_AS_SYSTEM_HEADERS "Exports library as system headers" OFF)
+message(STATUS "${projectPrefix}BUILD_AS_SYSTEM_HEADERS: ${${projectPrefix}BUILD_AS_SYSTEM_HEADERS}")
+
 option(${projectPrefix}BUILD_CXX_MODULES "Add C++ modules to the list of default targets" OFF)
 message(STATUS "${projectPrefix}BUILD_CXX_MODULES: ${${projectPrefix}BUILD_CXX_MODULES}")
 
+if(${projectPrefix}BUILD_AS_SYSTEM_HEADERS)
+    set(${projectPrefix}_AS_SYSTEM SYSTEM)
+endif()
+
 # project API settings
 function(cache_var_values name)
     set_property(CACHE ${projectPrefix}${name} PROPERTY STRINGS ${ARGN})

src/cmake/AddMPUnitsModule.cmake

@@ -70,6 +70,10 @@ function(add_mp_units_module name target_name)
         target_sources(
             ${target_name} PUBLIC FILE_SET HEADERS BASE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/include FILES ${ARG_HEADERS}
         )
+        target_include_directories(
+            ${target_name} ${${projectPrefix}_AS_SYSTEM} ${${projectPrefix}TARGET_SCOPE}
+            $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include> $<INSTALL_INTERFACE:include>
+        )
     endif()
 
     if(${projectPrefix}BUILD_CXX_MODULES)