Get rid of nasty GNU Make extensions.

Makefile

@@ -1,14 +1,16 @@
+.SUFFIXES: .c .o
+
 SRCS = boot.c
 LDFLAGS = -flto -T linker.ld -nostdlib -Os
 CFLAGS = -flto -ffreestanding -fno-builtin -fno-pie -Wall -std=c2x -m16 -nostdlib -Os -nostartfiles -r
 
-%.o: %.c
-	$(CC) $^ -o $@ $(CFLAGS)
+.c.o:
+	$(CC) $< -o $@ $(CFLAGS)
 
 bin: $(SRCS:.c=.o)
-	$(CC) $(LDFLAGS) -o bin $^
+	$(CC) $(LDFLAGS) -o bin $(SRCS:.c=.o)
 
 all: bin
 
 clean:
-	$(RM) $(SRCS:.c=.o) bin
+	rm -f $(SRCS:.c=.o) bin