Only pass -Wformat-signedness to versions of GCC that support it.

Fixes #3478

Signed-off-by: okhowang(王沛文) <okhowang@tencent.com>

CMakeLists.txt

@@ -165,7 +165,10 @@ if(CMAKE_COMPILER_IS_GNU)
         set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wlogical-op")
     endif()
     if (GCC_VERSION VERSION_GREATER 4.8 OR GCC_VERSION VERSION_EQUAL 4.8)
-        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wshadow -Wformat-signedness")
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wshadow")
+    endif()
+    if (GCC_VERSION VERSION_GREATER 5.0)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wformat-signedness")
     endif()
     set(CMAKE_C_FLAGS_RELEASE     "-O2")
     set(CMAKE_C_FLAGS_DEBUG       "-O0 -g3")

ChangeLog.d/format-signedness.txt

@@ -0,0 +1,3 @@
+Changes
+   * Only pass -Wformat-signedness to versions of GCC that support it. Reported
+     in #3478 and fix contributed in #3479 by okhowang.