configure: allow stp being installed in /

I have stp in standard paths: /usr/include and /usr/lib64. Allow that
by correct STP_CFLAGS + STP_LDFLAGS instead of STP_ROOT. Those are
empty when --with-stp is not passed.

configure is regenerated by autoconf too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Upstream libstp is no longer dependent on Boost so remove the
configure/Makefile code that adds Boost as a depdendency because We
don't need to support old versions of STP that needed Boost.
Merge pull request #174 from delcypher/allow_stp_installed_in_root

Allow stp installed in root