[patch] Use relative rpath so that the installation and build dirs are relocatable

[Rafael Espíndola]

The attached patch is based on a suggestion by Brad. It changes the build to use
a relative rpath. That way no patching is needed during installation
and both the
build and install directories are relocatable.

This is the same technique we use with configure+make.

Unfortunately for it to work on OS X we need 2.8.12. If that is not OK
we can probably
leave the apple branch commented for now with a note to enable it when we switch
to 2.8.12.

Cheers,
Rafael
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