xfsprogs: enable sparse checking with make C=1

xfsprogs: enable sparse checking with make C=1

Enable "make C=1" sparse checking when files get rebuilt.  To check
all files, run "make clean" first.

This is a bit simpler than redefining CC for the whole build, which
requires extra commandline definitions and apparently is enough of a
barrier that nobody's doing sparse checking.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>