c-find-decl-spots: Replace an arbitrary search limit by a parameter

c-find-decl-spots: Replace an arbitrary search limit by a parameter

The old arbitrary limit led to errors thrown whilst fontifying large areas of
syntactic whitespace containing "#undef", as in src/comp.c.

* lisp/progmodes/cc-engine.el (c-find-decl-spots): Replace the limit argument
to two calls of c-forward-syntactic-ws by the parameter `cfd-limit'.