When I installed libgc on mac, as you did, the files were installed to
/usr/local/Cellar/bdw-gc/. Then, when it came time to compile my code I had to run:
$ LIBGC=/usr/local/Cellar/bdw-gc/VERSION/ $ gcc ... -I$LIBGC/include/ ... $LIBGC/lib/libgc.a other.a ...
When you install libgc, its not included in your system path. You need to explicitly add it.
Also in my code I used:
CLICK HERE to find out more related problems solutions.