redoconf: move -Wl,-rpath flags to shlib.rc and add -Wl,-z,origin.

It was kind of ugly to have this kind of special flag directly in
link.od; now we detect whether it's available.  This also gives a
project the ability to override the flag in some other way if they
want.

As a result, rc/shlib.rc needs to be included in all.rc.od if you want
to create or link against shared libraries.

While we're here, also add -Wl,-z,origin on platforms where it works.
This is apparently needed for FreeBSD 8.0 at least, and probably other
platforms.

Reported-by: Wayne Scott <wsc9tt@gmail.com>

docs/cookbook/c/all.rc.od

@@ -2,6 +2,7 @@
 rc_include \
 	rc/CC.required.rc \
 	rc/CXX.rc \
+	rc/shlib.rc \
 	rc/libqt4.rc \
 	rc/libgtk2.rc \
 	rc/Wextra.rc \