fix  CMake can't find check_symbol_exists function.
On windows this problem doesn't appear.
Enviroment: OSX 0.13
cmake version 3.8.2
include(CheckSymbolExists)
if (ENABLE_SRV STREQUAL ON OR ENABLE_SRV STREQUAL AUTO)
+include(CheckSymbolExists)
if (WIN32)
set (RESOLV_LIBS Dnsapi)
set (MONGOC_HAVE_DNSAPI 1)
