configure: add the --libdir option

configure: add the --libdir option

This commit allows users/packagers to choose a lib directory to store
iproute2 lib files.

At the moment iproute2 ship lib files in /usr/lib and offers no way to
modify this setting. However, according to the FHS, distros may choose
"one or more variants of the /lib directory on systems which support
more than one binary format" (e.g. /usr/lib64 on Fedora).

As Luca states in commit a3272b93725a ("configure: restore backward
compatibility"), packaging systems may assume that 'configure' is from
autotools, and try to pass it some parameters.

Allowing the '--libdir=/path/to/libdir' syntax, we can use this to our
advantage, and let the lib directory to be chosen by the distro
packaging system.

Note that LIBDIR uses "\${prefix}/lib" as default value because autoconf
allows this to be expanded to the --prefix value at configure runtime.
"\${prefix}" is replaced with the PREFIX value in check_lib_dir().

Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David Ahern <dsahern@kernel.org>