tools: honour --libdir when it is passed to ./configure

tools: honour --libdir when it is passed to ./configure

Currently shared libraries are automatically installed into /usr/lib
or /usr/lib64, depending on the supplied --prefix value and
$(XEN_TARGET_ARCH). Some systems, like recent Debian and Ubuntu releases,
do not use /usr/lib64, but instead /usr/lib/x86_64-linux-gnu.

With this change, packagers can supply the desired location for shared
libraries on the ./configure command line. Packagers need to note that
the default behaviour on 64-bit Linux systems will be to install shared
libraries in /usr/lib, not /usr/lib64, unless a --libdir value is provided
to ./configure.

Additionally, the libfsimage plugins are now loaded explicitly from
$LIBDIR/fs, removing platform-based decision trees in code.

Signed-off-by: Matt Wilson <msw@amazon.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- resolve rejects in configure by rerunning autogen.sh. Dropped changes
to remove m4/default_lib.m4 and update m4/pkg.m4 since they cause LIBDIR=/lib
instead of /usr/lib. Reran ./autogen.sh after that too ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>