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 not redone by autoconf, since I do not have autoconf
2.60. I use 2.69, but it produces slightly different results. So I am
leaving this part up to you.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>