build: IWYU support added

src/CMakeLists.txt

@@ -26,6 +26,11 @@ project(mp-units
     LANGUAGES CXX
 )
 
+option(UNITS_AS_SYSTEM_HEADERS "Exports library as system headers" OFF)
+if(UNITS_AS_SYSTEM_HEADERS)
+    set(units_as_system SYSTEM)
+endif()
+
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")
 
 include(AddUnitsModule)

src/cmake/AddUnitsModule.cmake

@@ -28,7 +28,7 @@ cmake_minimum_required(VERSION 3.15)
 function(add_units_module name)
     add_library(mp-units-${name} INTERFACE)
     target_link_libraries(mp-units-${name} INTERFACE ${ARGN})
-    target_include_directories(mp-units-${name} INTERFACE
+    target_include_directories(mp-units-${name} ${units_as_system} INTERFACE
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
         $<INSTALL_INTERFACE:include>
     )

src/core/CMakeLists.txt

@@ -39,7 +39,7 @@ target_compile_features(mp-units-core INTERFACE cxx_std_20)
 target_link_libraries(mp-units-core INTERFACE
     gsl::gsl-lite
 )
-target_include_directories(mp-units-core INTERFACE
+target_include_directories(mp-units-core ${units_as_system} INTERFACE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
     $<INSTALL_INTERFACE:include>
 )