update configure defaults

update configure defaults

Currenly xfsprogs has two different sets of defauls for it's installation
layout.  One is in set in the configure.in file and used when running
./configure directly, and the second one is passed by make to ./configure
when configuring through make.

The defaults in the makefile are a lot better, as they install xfsprogs
into the / vs /usr split used by most distributions and enable multi-libs
when detected.

This patch changes the build system to also use the same defaults when
running ./configure.  To do this we get rid of some of the hacky
autoconf abuse we had in the past in xfsprogs and just use the normal
default sbindir and libdir variables for /usr/sbin an /usr/lib respecticly
and define new root_sbindir and root_libdir variables for those few
binaries installed into /.  The installation to / is disabled if the
user chooses a non-default exec prefix.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>