Pacify --enable-gcc-warnings when no window system is used.
These warnings found that subscript error, so they seem worthwhile.
* composite.c (char_composable_p): Simplify a bit.
* frame.c (x_set_frame_parameters): Add an IF_LINT.
* frame.c (x_set_horizontal_scroll_bars, x_set_scroll_bar_height):
* frame.h (FRAME_HAS_HORIZONTAL_SCROLL_BARS):
* window.c (set_window_scroll_bars):
Use USE_HORIZONTAL_SCROLL_BARS for simplicity.
* frame.h [! USE_HORIZONTAL_SCROLL_BARS]:
Ignore -Wsuggest-attribute=const.
* window.h (USE_HORIZONTAL_SCROLL_BARS): New macro.
(WINDOW_HAS_HORIZONTAL_SCROLL_BAR): Use it.