tools: allow configure time choice of libexec subdirectory.

tools: allow configure time choice of libexec subdirectory.

Currently we hardcode various paths such as $libexec/xen/{bin,boot},
however some downstreams (notably Debian) would like instead to
install things into $libexec/xen-X.Y/{bin,boot} as part of allowing
multiple versions of the tools packages to be installed.

Since this currently involves patching configure its a bit fiddly,
provide a configure option for the leaf dir instead, name it
--with-libexec-leaf-dir similar to the existing
--with-sysconfig-leaf-dir.

Rather than have the determination of the full path in both configure
and config/Paths.mk.in move it into configure only. Also for
consistency move the other LIBEXEC_* to configure, even though they
are only substituted into Paths.mk.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: 805508@bugs.debian.org
[ ijc -- removed stray ` ]