* configure.in: Add glyphs category to --enable-checking option.

* configure.in: Add glyphs category to --enable-checking option.
(GLYPH_DEBUG): Define if glyphs debugging is enabled.
* src/dispextern.h (GLYPH_DEBUG): Now defined in config.h if
enabled with --enable-checking=[all,glyphs] configure option.
Fix GLYPH_DEBUG usage assuming that it may be undefined,
adjust comments accordingly.
* src/dispnew.c: Fix GLYPH_DEBUG usage assuming that it may be
undefined, adjust comments accordingly.
* src/image.c: Likewise.
* src/scroll.c: Likewise.
* src/w32fns.c: Likewise.
* src/w32term.c: Likewise.
* src/xdisp.c: Likewise.
* src/xfaces.c: Likewise.
* src/xfns.c: Likewise.
* src/xterm.c: Likewise.