Be more consistent with Makefile variables

Makefile

@@ -1,6 +1,6 @@
 OUTPUT ?= .
 LIBNAME ?= libmtar
-OBJDIR ?= obj
+OBJ ?= obj
 SRC := src
 
 CC ?= gcc
@@ -9,14 +9,14 @@ CFLAGS ?= -O2 -Wall -Wextra
 CFLAGS := ${CFLAGS} -I.
 DESTDIR ?= /usr/local/lib
 
-OBJS := $(OBJDIR)/tar.o
+OBJS := $(OBJ)/tar.o
 
 build: $(OBJS)
 	@echo -- Creating $(LIBNAME).a
 	@mkdir -p $(OUTPUT)
 	$(AR) rcs $(OUTPUT)/$(LIBNAME).a $(OBJS)
 
-$(OBJDIR)/%.o: $(SRC)/%.c
+$(OBJ)/%.o: $(SRC)/%.c
 	@echo -- Compiling $^
 	@mkdir -p $(@D)
 	$(CC) $(CFLAGS) -o $@ -c $^
@@ -27,7 +27,7 @@ install:
 	cp $(OUTPUT)/$(LIBNAME).a $(DESTDIR)
 
 clean:
-	rm -f $(OBJDIR)/*.o
+	rm -f $(OBJ)/*.o
 	rm -f $(OUTPUT)/$(LIBNAME).a
 
 uninstall: