feature: [header only] fix CMake compile feature

Author: LanderlYoung

CMakeLists.txt

@@ -560,10 +560,15 @@ else()
   # to uncomment this, finally
 #  add_library(hwy INTERFACE)
 #  target_compile_definitions(hwy INTERFACE -DHWY_HEADER_ONLY)
-#  target_include_directories(hwy INTERFACE
-#          $<NOT HWY_WARNINGS_ARE_ERRORS:SYSTEM>
-#          $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
-#          $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+#  if (HWY_WARNINGS_ARE_ERRORS)
+#    target_include_directories(hwy INTERFACE
+#            $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
+#            $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+#  else()
+#    target_include_directories(hwy SYSTEM INTERFACE
+#            $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
+#            $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+#  endif ()
 #  target_link_libraries(hwy INTERFACE ${ATOMICS_LIBRARIES})
 
   add_library(hwy STATIC)
@@ -572,10 +577,15 @@ else()
     target_compile_features(hwy PRIVATE ${HWY_CXX_STD_TGT_COMPILE_FEATURE})
   endif ()
   target_compile_definitions(hwy PUBLIC -DHWY_HEADER_ONLY)
-  target_include_directories(hwy PUBLIC
-          $<NOT HWY_WARNINGS_ARE_ERRORS:SYSTEM>
-          $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
-          $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+  if (HWY_WARNINGS_ARE_ERRORS)
+    target_include_directories(hwy PUBLIC
+            $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
+            $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+  else ()
+    target_include_directories(hwy SYSTEM PUBLIC
+            $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
+            $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+  endif ()
   target_link_libraries(hwy PRIVATE ${ATOMICS_LIBRARIES})
   target_sources(hwy PRIVATE
 #          hwy/abort.cc