cmake: Specify -std=c++11

gcc 5.1 will complain (and refuse to compile) if you use 'auto' in the
C++11 sense but haven't asked for C++11.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 07deeeb..b5c63c6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,7 +20,7 @@
     set(COMMON_COMPILE_FLAGS "-Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers")
     set(COMMON_COMPILE_FLAGS "${COMMON_COMPILE_FLAGS} -fno-strict-aliasing -fno-builtin-memcmp")
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99 ${COMMON_COMPILE_FLAGS}")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${COMMON_COMPILE_FLAGS}")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${COMMON_COMPILE_FLAGS} -std=c++11")
     if (UNIX)
         set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fvisibility=hidden")
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden")