diff --git a/CMakeLists.txt b/CMakeLists.txt
index f5ba40b7..f5b576d3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,6 +5,14 @@ project(GmSSL)
 SET(EXECUTABLE_OUTPUT_PATH ${PROJECT_BINARY_DIR}/bin)
 SET(LIBRARY_OUTPUT_PATH ${PROJECT_BINARY_DIR}/lib)
 
+if (CMAKE_VERSION VERSION_LESS "3.1")
+  if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
+    set (CMAKE_C_FLAGS "-std=gnu99 ${CMAKE_C_FLAGS}")
+  endif ()
+else ()
+  set (CMAKE_C_STANDARD 99)
+endif ()
+
 #set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -mrdrnd -mrdseed")
 
 include_directories(include)