Prevent libtool from adding annoying stuff to RPATH

--- cairo/configure.orig	2018-10-19 22:20:08.000000000 +0200
+++ cairo/configure	2019-09-04 17:53:00.927539686 +0200
@@ -16592,6 +16592,7 @@
 	    esac
 	    ;;
 	esac
+hardcode_libdir_flag_spec=
 	;;
 
       lynxos*)
