Correct fontification of macro "IF_LINT" in our C sources.

[Alan Mackenzie]

Hello, Emacs.

I've just updated .dir-locals (in the master branch) such that
occurrences of the macro "IF_LINT" no longer throw the fontification
of declarations off.  This uses a relatively new feature of CC Mode,
"noise macros".

This currently asks you to confirm the acceptance of these "risky" local
variables each time you visit one of Emacs's C files.  The irritation
can be ameliorated by accepting them once and for all with "!" rather
than having to type "y" for each file.

There are certainly other such "noise macros" (macros which expand to
nothing) which break fontification.  Feel free to expand the entry for C
files in .dir-locals, or alternatively let me know and I'll do it.
"Noise macros" are documented in the CC Mode manual.

-- 
Alan Mackenzie (Nuremberg, Germany).