build: MP_UNITS_BUILD_AS_SYSTEM_HEADERS restored (but renamed)

CMakeLists.txt

@@ -42,6 +42,8 @@ enable_ccache(BASE_DIR ${PROJECT_SOURCE_DIR})
 option(${projectPrefix}DEV_IWYU "Enables include-what-you-use" OFF)
 
 if(${projectPrefix}DEV_IWYU)
+    set(${projectPrefix}BUILD_AS_SYSTEM_HEADERS ON)
+
     include(include-what-you-use)
     enable_iwyu(
         MAPPING_FILE "${PROJECT_SOURCE_DIR}/.mp-units.imp"

docs/getting_started/installation_and_usage.md

@@ -223,6 +223,14 @@ tools.build:compiler_executables={"c": "gcc-12", "cpp": "g++-12"}
 
 ### CMake options
 
+[`MP_UNITS_BUILD_AS_SYSTEM_HEADERS`](#MP_UNITS_BUILD_AS_SYSTEM_HEADERS){ #MP_UNITS_BUILD_AS_SYSTEM_HEADERS }
+
+:   [:octicons-tag-24: 2.2.0][as system headers support] · :octicons-milestone-24: `ON`/`OFF` (Default: `OFF`)
+
+    Exports library as system headers.
+
+    [as system headers support]: https://github.com/mpusz/mp-units/releases/tag/v2.2.0
+
 [`MP_UNITS_BUILD_CXX_MODULES`](#MP_UNITS_BUILD_CXX_MODULES){ #MP_UNITS_BUILD_CXX_MODULES }
 
 :   [:octicons-tag-24: 2.2.0][build_cxx_modules support] · :octicons-milestone-24: `ON`/`OFF` (Default: `OFF`)

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)