The Solaris linker uses "-R" instead of "--rpath". The GNU linker can use
either. The Makefile for Solaris (SunOS) has a test for GNU linker, and
selects the appropriate switch. But the test relies on which "ld" is found
in the path, and it might not match what gcc uses.
Presumably you have GNU "ld" installed, so try adding it earlier than
/usr/ccs/bin/ in your PATH before running make, so that the Makefile finds
the correct version.
Alternatively, edit Makefile.SunOS and change "--rpath" to "-R".
But first do what Paul suggested and re-run configure.