Use $ORIGIN in rpath instead of hard-coding

[ljacobson64]

Right now, we hard-code the rpath into our libraries as /opt/Trelis-17.1/bin/plugins/svalinn. This hard-coding means that the plugin will not work with different versions of Trelis. One solution to this problem would be to set the rpath to be $ORIGIN, which apparently resolves to wherever the executable or library is at runtime.

See this article: https://medium.com/@nehckl0/creating-relocatable-linux-executables-by-setting-rpath-with-origin-45de573a2e98