configure: correctly set rpath for linking with CLN ...

configure: correctly set rpath for linking with CLN ...

... as to not break tests for libreadline and dlopen().

Problem:

When using GCC 4.6 the configure script fails to find libreadline and
libdl, although both libraries are definitely installed. See
http://www.ginac.de/pipermail/ginac-list/2012-January/001868.html for
more details.

Reason:

Apparently GCC 4.6 dislikes the -R/the/path switch (libtool way to say
-Wl,-rpath,/the/path in a cross platform manner). Previous versions of
GCC used to ignore the -R switch, however, GCC 4.6 errors out instead.
Thus the configure script fails to detect readline and dlopen.

Solution:

Set the rpath (for linking with CLN) using the compiler friendly syntax
(i.e. -Wl,-rpath -Wl,/the/path instead of -R/the/path).