Simplify logic for shared_ptr and unordered_map

Just check if the headers exists during configure.

Compilers with support for c++11 will use the one
part of the standard, the others will use the
ones in tr1 (that would be for g++/clang++)

Change-Id: I18d88eff0927fc255e4cc1ce66eb4da42d45930d
Reviewed-on: http://review.couchbase.org/29967
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>