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
@@ -10432,6 +10432,7 @@
       else
         ld_shlibs=no
       fi
+hardcode_libdir_flag_spec=
       ;;
 
     netbsd*)