...
WriteMakefile(
    NAME	=> 'XML::Parser::Expat',
    C         => ['Expat.c'],
    LIBS	=> $libs,
    XSPROTOARG => '-noprototypes',
    VERSION_FROM => 'Expat.pm',
    LDDLFLAGS => $Config{lddlflags}." -Wl,-rpath,$ENV{ORIGIN}/../lib",    # <-- add this
    @extras
);