diff --git a/CMakeLists.txt b/CMakeLists.txt
index 08672e8..376bc7c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -13,7 +13,11 @@ option(USE_OPENSSL "set OFF to build without OpenSSL" ON)
 
 add_library(simple-web-server INTERFACE)
 
-target_include_directories(simple-web-server INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
+if(CMAKE_SOURCE_DIR STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
+    target_include_directories(simple-web-server INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
+else()
+    target_include_directories(simple-web-server SYSTEM INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
+endif()
 
 find_package(Threads REQUIRED)
 target_link_libraries(simple-web-server INTERFACE Threads::Threads)
@@ -24,7 +28,7 @@ if(USE_STANDALONE_ASIO)
     if(NOT ASIO_PATH)
         message(FATAL_ERROR "Standalone Asio not found")
     else()
-        target_include_directories(simple-web-server INTERFACE ${ASIO_PATH})
+        target_include_directories(simple-web-server SYSTEM INTERFACE ${ASIO_PATH})
     endif()
 else()
     find_package(Boost 1.53.0 COMPONENTS system thread REQUIRED)