Fix libtool RPATH outsmarting hack for external/librevenge

After the blind fix attempt of 490f07cf "Extend
libtool RPATH outsmarting hack to external/librevenge", appears that
<https://ci.libreoffice.org/job/lo_daily_update_gandalf/596/> also needs
runpath_var=LD_RUN_PATH to be reset.  (See also how
<https://src.fedoraproject.org/cgit/rpms/librevenge.git/tree/librevenge.spec
?id=4960d4c6c190885b20f56ce9ee1ad2ad92b87021#n46> addresses the same problem for
Fedora builds of librevenge.)

Change-Id: I5cff145605cd05a8b87360c1edc3574e3364139b
Reviewed-on: https://gerrit.libreoffice.org/68800
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/librevenge/rpath.patch

 --- configure
 +++ configure
-@@ -14983,6 +14983,7 @@
+@@ -14983,6 +14983,8 @@
  	    esac
  	    ;;
  	esac
 +hardcode_libdir_flag_spec_CXX=
++runpath_var=
  	;;
  
        lynxos*)