Treat SYSTEM include directories as such

Author: AMDmi3

contrib/gdk-pixbuf/CMakeLists.txt

@@ -12,7 +12,7 @@ if(AVIF_BUILD_GDK_PIXBUF)
 
             target_link_libraries(pixbufloader-avif PUBLIC ${GDK_PIXBUF_LIBRARIES} avif)
             target_link_directories(pixbufloader-avif PUBLIC ${GDK_PIXBUF_LIBRARY_DIRS})
-            target_include_directories(pixbufloader-avif PUBLIC ${GDK_PIXBUF_INCLUDE_DIRS})
+            target_include_directories(pixbufloader-avif SYSTEM PUBLIC ${GDK_PIXBUF_INCLUDE_DIRS})
 
             pkg_get_variable(GDK_PIXBUF_MODULEDIR gdk-pixbuf-2.0 gdk_pixbuf_moduledir)
             string(REPLACE ${GDK_PIXBUF_PREFIX} ${CMAKE_INSTALL_PREFIX} GDK_PIXBUF_MODULEDIR ${GDK_PIXBUF_MODULEDIR})