Include CheckFunctionExists to CMakeLists.txt.

Without this change, a "cmake ." for me would error with...

    Unknown CMake command "CHECK_FUNCTION_EXISTS".

Change-Id: Ic970d133eae2e8100a27ff0d2a9c3d79a9ef19a2
Reviewed-on: http://review.couchbase.org/39818
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Steve Yen <steve.yen@gmail.com>