Add pkg-config gc flags to llvm Makefile

This makes building work for systems which have gc installed in a
non-standard location.  The invocation was pulled from
https://github.com/schani/clojurec/pull/2.  I'm not sure whether or not
the --libs is needed; I have the relevant library path in LD_RUN_PATH
and LD_LIBRARY_PATH, but I think the --libs is needed if this is not the
case.