Move DEPFLAGS from Makefile to config.mk

Makefile

@@ -7,7 +7,6 @@ all: sxiv
 include config.mk
 
 CPPFLAGS += -I. -DVERSION=\"$(VERSION)\" -DHAVE_GIFLIB=$(HAVE_GIFLIB) -DHAVE_LIBEXIF=$(HAVE_LIBEXIF)
-DEPFLAGS := -MMD -MP
 
 LDLIBS := -lImlib2 -lX11 -lXft
 

config.mk

@@ -4,6 +4,7 @@ MANPREFIX := $(PREFIX)/share/man
 CC        ?= gcc
 CFLAGS    += -std=c99 -Wall -pedantic
 CPPFLAGS  += -I/usr/include/freetype2 -D_XOPEN_SOURCE=700
+DEPFLAGS  := -MMD -MP
 LDFLAGS   += 
 
 # autoreload backend: inotify/nop