Build failure: -Wno-unused-const-variable DNE on old GCC

[David Laight]

From: Michael Ellerman
> Sent: 08 January 2016 02:17
> > I doubt you plan to fix all those. So making -Werror configurable seems like
> > the only way forward, then. (Glad you agreed!)
> 
> Yep, I'm happy to make Werror configurable.
> 
> cxl can probably just use the existing PPC_WERROR.

Personally I think the development builds should be done with
both error -Werror and higher levels of warnings, even if the
ones that users tend to do ignore more warnings.

Clearly some warnings that some versions of gcc have included
in -Wall are just a PITA (like warning for __DATE__ and __TIME__).

The kernel headers do need fixing so that they pass compilation
with options like -Wwrite-strings (linux/mm.h doesn't).

	David