CXX-204 Use vendored timegm implementation if platform does not offer it

(cherry picked from commit 5bf477aa3dee9e82c9c2286a9ad8341cd71dfe97)

Conflicts:
	SConstruct
	src/mongo/util/time_support.cpp
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