Remove 'add_definitions(-Wno-gnu-zero-variadic-macro-arguments)'.

CMakeLists.txt

@@ -502,7 +502,6 @@ if (${CMAKE_BUILD_TYPE} MATCHES "[Dd]ebug")
 endif()
 if (CIVETWEB_ENABLE_HTTP2)
   add_definitions(-DUSE_HTTP2)
-  add_definitions(-Wno-gnu-zero-variadic-macro-arguments)
 endif()
 if (CIVETWEB_ENABLE_IPV6)
   add_definitions(-DUSE_IPV6)