dtc: Enable more warnings

[Jon Loeliger]

> This patch turns on a bunch of extra gcc warnings, most of which are
> probably a good idea.  Of the new warnings -Wnested-externs and
> -Wstrict-prototypes need no code changes, we're already warning-clean.
> The remaining one, -Wmissing-prototypes requires trivial changes in
> some of the tests (making functions local).
> 
> This patch also rearranges the warnings flags into a separate make
> variable for convenience, and turns on -Werror, to really encourage
> people to keep the code warning-clean.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Well, I'd like to apply this patch.  Doing so is actually effective!

$ make check 2>&1 | less
        CHK version_gen.h
         CC tests/testutils.o
cc1: warnings being treated as errors
tests/testutils.c: In sigint_:
tests/testutils.c:44: error: unused 
tests/testutils.c:44: error: unused 
tests/testutils.c: In test_:
tests/testutils.c:52: error: unused 
tests/testutils.c: In check_:
tests/testutils.c:113: error: comparison between signed and unsigned
tests/testutils.c:117: error: comparison between signed and unsigned
make: *** [tests/testutils.o] Error 1
propertyfunction argcparameter initfunction ucparameter siparameter handlerfunction