std/bslhdrs - declare only relevant names

Components in 'bsl+bslhdrs' and 'bsl+stdhdrs' have bent to the will of
backwards compatibility and declared names that do not exist in the underlying
standard C++ headers.  This causes issues in strictly standard-compliant
headers since name declarations may clash or may not be found.

This commit only declares the appropriate names, preserving the old behavior
if the 'BSL_OVERRIDES_STD' define is present (which it is in legacy Bloomberg
code).
Fix building with 'xcode 5'.

In particular, include <sys/time.h> instead of <time.h> to get
platform-specific types related to time (making Darwin more like FreeBSD and
AIX).