--- CMakeLists.txt.orig	2016-04-08 02:04:21 UTC
+++ CMakeLists.txt
@@ -213,7 +213,7 @@ else()
 
   # On FreeBSD 64 math.h uses unguarded C11 extension, which taints clang
   # 3.4.1 used there.
-  if(CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
+  if(CMAKE_SYSTEM_NAME STREQUAL "FreeBSD" AND CMAKE_C_COMPILER_ID MATCHES "Clang")
     add_definitions(-Wno-c11-extensions)
   endif()
 endif()