build: initialize stack variables to zero by default

build: initialize stack variables to zero by default

Newer versions of gcc and clang can include the ability to zero stack
variables by default.  Let's enable it so that we (a) reduce the risk of
writing stack contents to disk somewhere and (b) try to reduce
unpredictable program behavior based on random stack contents.  The
kernel added this 6 years ago, so I think it's mature enough for
xfsprogs.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reluctantly-Reviewed-by: Christoph Hellwig <hch@lst.de>