This compiler defines shared_ptr in std::tr1 namespace, but
for this <tr1/memory> is to be included. Further, this compiler
also does have <memory> header which confused previous shared
pointer check.

Simplified logic around defines now, so currently we've got:
- CERES_TR1_MEMORY_HEADER defined if <tr1/memory> is to be
  used for shared_ptr, otherwise <memory> is to be used.
- CERES_TR1_SHARED_PTR defined if shared_ptr is defined in
  std::tr1 namespace, otherwise it's defined in std namespace.

All the shared_ptr checks are now moved to own file FindSharedPtr
which simplifies main CMakeLists.

Change-Id: I558a74793baaa0bd088801910a356be4ef17c31b