The build_ext command does accept a handy --rpath option to encode an
RPATH in the built extension modules.  However RPATH is superseded by
RUNPATH since the former can not be overwritten by the LD_LIBRARY_PATH
environment varialbe, while the later can.  While most linkers will add
a RUNPATH automatically when you ask for an RPATH, GNU ld does not do
this.  Therefore this patch does detect if GNU ld is used and if so will
use the --enable-new-dtags option which will add the RUNPATH.