Fix some portability problems with 'inline'.

Fix some portability problems with 'inline'.

* dispextern.h (window_box, window_box_height, window_text_bottom_y)
(window_box_width, window_box_left, window_box_left_offset)
(window_box_right, window_box_right_offset): Declare extern.
Otherwise, these inline functions do not conform to C99 and
are miscompiled by Microsoft compilers.  Reported by Eli Zaretskii in
<http://lists.gnu.org/archive/html/emacs-devel/2011-11/msg00084.html>.
* intervals.c (adjust_intervals_for_insertion)
(adjust_intervals_for_deletion): Now extern, because otherwise the
extern inline functions 'offset_intervals' couldn't refer to it.
(static_offset_intervals): Remove.
(offset_intervals): Rewrite using the old contents of
static_offset_intervals.  The old version didn't conform to C99
because an extern inline function contained a reference to an
identifier with static linkage.