Define _GNU_SOURCE to expose qsort_r().

Makefile

@@ -1,7 +1,7 @@
 OFLAGS += -Wall -Wno-unused -O3 -DNDEBUG
 GFLAGS += -Wall -Wno-unused -g
 PFLAGS += $(OFLAGS) -pg
-CFLAGS += -I/opt/local/include
+CFLAGS += -D_GNU_SOURCE -I/opt/local/include
 LDLIBS += -L/opt/local/lib
 LDLIBS += -lgc -lm