Note linux-libc-dev build dependency (for ext2_fs.h).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Remove dependency on linux/ext2_fs.h

Due to a missing header in (some versions of) linux/ext2_fs.h,
configure's AC_CHECK_HEADERS test fails for this header, making it
appear "not found", and causing Linux-attr support to be disabled in bup
for no good reason.

Given that all the flags bup needs are available in linux/fs.h, we can
safely remove the requirement for linux/ext2_fs.h altogether, to work
around the problem.

Signed-off-by: Yung-Chin Oei <yungchin@yungchin.nl>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Makefile: clean config/ during clean.

Signed-off-by: Yung-Chin Oei <yungchin@yungchin.nl>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Explicitly select the "du" block size in t/test.sh.

Since du's default output can be affected by several environment
variables (DU_BLOCK_SIZE, BLOCK_SIZE and BLOCKSIZE), be explicit about
what we want via -B 1024.

Thanks to Michael Ekstrand <michael@elehack.net> for the original
report, and help with the fix.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>