CC Mode: Add newish AWK Mode facilities, as used in gawk-4.

CC Mode: Add newish AWK Mode facilities, as used in gawk-4.

* lisp/progmodes/cc-awk.el (c-awk-font-lock-invalid-namespace-separators):
New function.
(c-awk-context-expand-fl-region): New function.
(awk-font-lock-keywords): Enhance handling of function declarations to include
:: tokens.  Fontify new system variable names FPAT, FUNCTAB, PREC, ROUNDMODE,
SYNTAB.  Fontify new keywords BEGINFILE and ENDFILE.  Fontify new system
functions asorti, dcngettext, isarray, patsplit, typeof.  Fontify the new
directives @include, @load, @namespace.  Call
c-awk-font-lock-invalid-namespace-separators as a matcher.

* lisp/progmodes/cc-fonts.el (top level): No longer require 'cc-awk.

* lisp/progmodes/cc-langs.el (c-before-context-fontification-functions): Give
AWK the value c-awk-context-expand-fl-region rather than nil.

* lisp/progmodes/cc-mode.el (top level): Declare awk-mode-syntax-table as a
variable.