CXX-204 Add missed timegm implementation files from previous commit
CXX-236 Backport server r2.6.1..r2.6.2 changes
CXX-253 Include-what-you-use for make_shared

Older boost doesn't include make_shared.hpp in smart_ptr.hpp, so the
include of smart_ptr.hpp in pch.h doesn't make make_shared.hpp available
in the replica_set_monitor files.
CXX-254 Remove reference to deleted SConscript file
CXX-268 Backport CXX-150 Export Date_t and some auxilliary functions

(cherry picked from commit c05e8ff713afd3ebc07219f4ba3348df5b8cc432)
CXX-273 Backport server r2.6.2..r2.6.3 changes
Merge branch '26compat' of github.com:mongodb/mongo-cxx-driver into 26compat