Motivation
----------
Some linuxes (like Fedora) actually multiarch. And it means that
libgc.so.1 not always installed into /usr/lib. It is better to entrust
this job to ld and system configuration for it.
Modification
------------
Use relative path for Boehm GC on 'other' OS (and linux in particular)
Result
------
It is possible now to build nim with --gc:boehm on linux