Fix -soname issue with Mac linkers

[run4flat]

This may have been fixed upstream, but at any rate, on Macs I cannot compile a shared version of the library. It looks like a mere change to the linker options is sufficient to fix it, though.

[run4flat]

This ticket is no longer explicit enough to be useful (the link is bad, so I'm not even sure what the proposed solution would have been, let alone the exact problem). Bah.