Made Makefile POSIX compliant

Makefile

@@ -2,20 +2,20 @@ all: sxiv
 
 VERSION = git-20110606
 
-CC?=gcc
-DESTDIR?=
-PREFIX?=/usr/local
-CFLAGS+= -Wall -pedantic -DVERSION=\"$(VERSION)\"
-LDFLAGS+= 
-LIBS+= -lX11 -lImlib2
+CC = gcc
+DESTDIR =
+PREFIX = /usr/local
+CFLAGS = -Wall -pedantic -DVERSION=\"$(VERSION)\"
+LDFLAGS =
+LIBS = -lX11 -lImlib2
 
-SRCFILES=$(wildcard *.c)
-OBJFILES=$(SRCFILES:.c=.o)
+SRC = image.c main.c options.c thumbs.c util.c window.c
+OBJ = $(SRC:.c=.o)
 
-sxiv:	$(OBJFILES)
+sxiv:	$(OBJ)
 	$(CC) $(LDFLAGS) -o $@ $^ $(LIBS)
 
-%.o: %.c Makefile config.h
+.c.o: Makefile config.h
 	$(CC) $(CFLAGS) -c -o $@ $<
 
 install: all
@@ -25,10 +25,4 @@ install: all
 	chmod 644 $(DESTDIR)$(PREFIX)/share/man/man1/sxiv.1
 
 clean:
-	rm -f sxiv *.o
-
-tags: *.h *.c
-	ctags $^
-
-cscope: *.h *.c
-	cscope -b
+	rm -f $(OBJ) sxiv