Add SYSTEM property to all dependencies

Add SYSTEM property to all `add_subdirectory` commands that add
external dependencies

CMakeLists.txt

@@ -176,9 +176,9 @@ if (PYTHON STREQUAL INSTALL)
     # Relies on undocumented behaviour. EXCLUDE_FROM_ALL is used to prevent install commands
     # inside the easyloggingpp CMakeLists from executing and subsequently failing with a permission error,
     # making it impossible to install the Python package as a normal user.
-    add_subdirectory("lib/easyloggingpp" EXCLUDE_FROM_ALL)
+    add_subdirectory("lib/easyloggingpp" EXCLUDE_FROM_ALL SYSTEM)
 else ()
-    add_subdirectory("lib/easyloggingpp")
+    add_subdirectory("lib/easyloggingpp" SYSTEM)
 endif ()
 set( CMAKE_BUILD_TYPE ${CMAKE_BUILD_TYPE_COPY} )
 
@@ -199,7 +199,7 @@ endif()
 add_subdirectory("cfg")
 
 if (PYTHON)
-    add_subdirectory("lib/pybind11")
+    add_subdirectory("lib/pybind11" SYSTEM)
     add_subdirectory("src/python_bindings")
 endif()