build: clean up Makefile; enforce C99 with GNU extensions

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I023a29897332377c4e8c116ab4d0a67b6a6a6964

Makefile

@@ -2,23 +2,40 @@ PREFIX ?= /usr
 BINDIR ?= $(PREFIX)/bin
 CC ?= gcc
 
-CFLAGS ?= -O2 -Wall -Wextra -Wpedantic
+CFLAGS ?= -O2 -Wall -Wextra -Wpedantic -std=gnu99 -I./include
 LDFLAGS ?= -Wl,-z,relro,-z,now -Wl,-z,noexecstack -Wl,-z,separate-code -pie
 LDLIBS ?= -lsystemd
 
-SRC = main.c
+SRCDIR = src
+OBJDIR = obj
+
+SRCS = $(wildcard $(SRCDIR)/*.c)
+OBJS = $(patsubst $(SRCDIR)/%.c,$(OBJDIR)/%.o,$(SRCS))
+
 BIN = sin
 
-.PHONY: all clean install test
+.PHONY: all clean install test dirs
 
-all: $(BIN)
+all: dirs $(BIN)
 
-$(BIN): $(SRC)
-	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(SRC) $(LDLIBS)
+dirs:
+	@mkdir -p $(OBJDIR)
+
+fmt:
+	clang-format -i src/*.c
+
+$(BIN): $(OBJS)
+	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)
+
+$(OBJDIR)/%.o: $(SRCDIR)/%.c
+	$(CC) $(CFLAGS) -c -o $@ $<
 
 install: $(BIN)
 	install -d $(DESTDIR)$(BINDIR)
 	install -m 0755 $(BIN) $(DESTDIR)$(BINDIR)/$(BIN)
 
 clean:
-	-rm -f $(BIN) test_sin
+	-rm -rf $(OBJDIR) $(BIN)
+
+test: $(BIN)
+	./$(BIN) --version