Support C++ compilers without C99 __func__

There is no point of repeating the function name anyway
because gtest already printed it there..

Change-Id: I43eba5f45b9e6d395912782a436f2aceccd32a94
Reviewed-on: http://review.couchbase.org/25228
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>