Makefile: ensure CONF_USR_DIR honours the libdir config

Makefile: ensure CONF_USR_DIR honours the libdir config

Following commit cee0cf84bd32 ("configure: add the --libdir option"),
iproute2 lib directory is configurable using the --libdir option on the
configure script. However, CONF_USR_DIR does not honour the configured
lib path in its default value.

This fixes the issue simply using $(LIBDIR) instead of $(PREFIX)/lib.
Please note that the default value for $(LIBDIR) is exactly
$(PREFIX)/lib, so this does not change the default value for
CONF_USR_DIR.

Fixes: 0a0a8f12fa1b ("Read configuration files from /etc and /usr")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>