add goal for gdc

makefile

@@ -4,8 +4,9 @@ DMD_COMMON_FLAGS := -dip25 -w $(INCLUDE_PATHS)
 DMD_FLAGS := -O -inline $(DMD_COMMON_FLAGS)
 DMD_TEST_FLAGS := -unittest -g $(DMD_COMMON_FLAGS)
 LDC_FLAGS := -g -w -oq $(INCLUDE_PATHS)
+GDC_FLAGS := -g -w -oq $(INCLUDE_PATHS)
 
-.PHONY: dmd ldc test
+.PHONY: dmd ldc gdc test
 
 dmd: bin/dfmt
 
@@ -13,6 +14,9 @@ ldc: $(SRC)
 	ldc2 $(LDC_FLAGS) $^ -ofbin/dfmt
 	-rm -f *.o
 
+gdc: $(SRC)
+	gdc $(GDC_FLAGS) $^ -obin/dfmt
+
 test: bin/dfmt-test
 	cd tests && ./test.sh