MB-9645: Workaround for broken c++ compilers

<unordered_map> should be part of C++11, but the clang 4.1.0
defines __cplusplus claiming it support it, but doesn't contain
the header..

Change-Id: I8503d1fdb996b393d9f40d1ef8821afbf6930ba0
Reviewed-on: http://review.couchbase.org/30665
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>