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:
#include "gc.h"
And not <gc/gc.h>.
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