[Rivet] --no-as-needed only exists for ELF linkers

[James Monk]

Hi,

The linker flag --no-as-needed has been added to the rivet-buildplugin script recently.  As far as I know, this only exists on Linux, certainly it causes my linker to fail on OS X.  

Can we add a check for OS X as already occurs for the shared library flags, and revert to the old behaviour in that case?

cheers,

James