hp_list: Modularize build system

It is straightforward to extend. For example, if we are about to add
"release" build, we can add the following:

release: CFLAGS += -O2 -DNDEBUG
release: $(BIN)

Author: jserv

hp_list/Makefile

@@ -1,5 +1,16 @@
-all:
-	$(CC) -Wall -o list main.c -lpthread -g -fsanitize=thread
+CC ?= gcc
+CFLAGS = -Wall -g
+CFLAGS += -fsanitize=thread
+LIBS = -lpthread
+BIN = list
+
+all: list
+
+$(BIN): main.c
+	$(CC) $(CFLAGS) -o $@ $< $(LIBS)
+
+all: CFLAGS += -O2
+all: $(BIN)
 
 indent:
 	clang-format -i *.[ch]