diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7f15337..71714fe 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,14 +1,17 @@
 cmake_minimum_required(VERSION 2.8)
 
-# enable C99 mode
-if(CMAKE_VERSION VERSION_LESS "3.1")
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
-else()
-    set (CMAKE_C_STANDARD 99)
-endif()
-
 project(ttyd)
 
+if(CMAKE_VERSION VERSION_LESS "3.1")
+    if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu99")
+    else()
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
+    endif()
+else()
+    set(CMAKE_C_STANDARD 99)
+endif()
+
 set(LIBWEBSOCKETS_MIN_VERSION 1.7.0)
 set(SOURCE_FILES src/server.c src/http.c src/protocol.c src/utils.c)