Fix gcc build problem

Fixed gcc-related build problem resulting from `-errwarn=%all` switch.

Fixes #3012.

CMakeLists.txt

@@ -298,7 +298,9 @@ endif ()
 
 if (LIBZMQ_WERROR)
   zmq_check_cxx_flag_prepend ("-Werror")
-  zmq_check_cxx_flag_prepend ("-errwarn=%all")
+  if (NOT "${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+    zmq_check_cxx_flag_prepend ("-errwarn=%all")
+  endif()
 endif ()
 
 if (CMAKE_SYSTEM_PROCESSOR MATCHES "^sparc")