Always define 'define-fringe-bitmap'

Always define 'define-fringe-bitmap'

* lisp/cus-start.el: Test 'fringe-bitmaps' to see if fringe.c was compiled.

* lisp/fringe.el: Use lexical-binding.
(define-fringe-bitmap): Provide a fallback implementation.

* lisp/progmodes/flymake.el (flymake-double-exclamation-mark):
Define unconditionally.

* lisp/progmodes/gdb-mi.el (define-fringe-bitmap): Don't declare any more.
(breakpoint, hollow-right-triangle): Define unconditionally.