Remove duplicated ${DESTDIR}.

Both Makefile.in and install.sh prepended ${DESTDIR} to ${PREFIX} so the
installation prefix became ${DESTDIR}/${DESTDIR}/${PREFIX}.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #290 : omasanori/cargo/makefile, r=alexcrichton

Both Makefile.in and install.sh prepended `${DESTDIR}` to `${PREFIX}` so the installation prefix became `${DESTDIR}/${DESTDIR}/${PREFIX}`. Also, the script assumes that `${DESTDIR}` ends with a slash or `${PREFIX}` starts with it so I appended a slash to `${DESTDIR}`.