Fix Makefile depend file usage

Creating the .depend file requires the config.h file to exist and
suppress make's complaints if the .depend file doesn't exist when
initially parsing the makefile.

Makefile

@@ -1,4 +1,4 @@
-VERSION = 1.3
+VERSION = git-20141025
 
 PREFIX    = /usr/local
 MANPREFIX = $(PREFIX)/share/man
@@ -14,7 +14,8 @@ OBJ = $(SRC:.c=.o)
 
 all: sxiv
 
-$(OBJ): Makefile config.h
+$(OBJ): Makefile
+$(OBJ) .depend: config.h
 
 depend: .depend
 
@@ -22,7 +23,7 @@ depend: .depend
 	rm -f ./.depend
 	$(CC) $(CFLAGS) -MM $^ >./.depend
 
-include .depend
+-include .depend
 
 .c.o:
 	$(CC) $(CFLAGS) $(CPPFLAGS) -DVERSION=\"$(VERSION)\" -c -o $@ $<