[RFA 0/6] Add -Wunused-but-set-parameter and -Wunused-but-set-variable

[Tom Tromey]

I happened to notice an unused variable in the newly minted Rust code,
so I decided to take a look at adding some warnings to prevent this.

This series adds -Wunused-but-set-parameter and
-Wunused-but-set-variable to the build.  First come the patches to
remove all the warnings, and the final patch enables the compiler
flags.

I've split up the patches for easier review, and according to the type
of patch.  I think they're generally self-explanatory.

I built and regression tested this using --enable-targets=all on
x86-64 Fedora 23.  However, I could not update the various nat-*
files, so there are probably unfixed warnings lurking there.

Tom