Fix glibc feature macro handling for timegm

Fix glibc feature macro handling for timegm

The way strptime is activated using feature macros, _DEFAULT_SOURCE
(successor to _BSD_SOURCE) is disabled implicitly, so timegm is
hidden.  Defining _DEFAULT_SOURCE at the same time as the other
feature macros solves this, and removes the need for the
TIMEGM_NEEDS_BSD configure macro.

This avoids an implicit declaration of timegm in src/bind.c, and build
failures with future compilers.