Port to clang 3.7.0 on x86-64

Port to clang 3.7.0 on x86-64

* configure.ac: Use AS_IF so that gl_WARN_ADD’s prerequisites are
not done conditionally.  This helps clang, which needs
-Wunknown-warning-option later when configured with warnings.
* src/editfns.c (invalid_time): Now _Noreturn, since clang isn’t
smart enough to figure this out on its own if warnings are enabled.
(lisp_time_struct): Redo for clarity, and to pacify clang.
* src/xfns.c (x_real_pos_and_offsets) [USE_XCB]: Don’t use
uninitialized locals.  This avoids undefined behavior and pacifies
clang.