Add flags to support compilation with Homebrew libgc.

src/Makefile

@@ -1,5 +1,5 @@
-CFLAGS = -I/opt/local/include -std=gnu99 -Wall -Wno-unused-label -Wno-unused-function -g
-LDLIBS = -L/opt/local/lib -lgc -lm
+CFLAGS = -I/opt/local/include -I/opt/homebrew/include -std=gnu99 -Wall -Wno-unused-label -Wno-unused-function -g
+LDLIBS = -L/opt/local/lib -L/opt/homebrew/lib -lgc -lm
 TREE = -t
 
 MAIN = ccmeta