Support out-of-source builds

Fixes issue #167. Long after closing this issue (kind of wontfix) we have switched to
GNU make and can thus use its features like VPATH.

Makefile

@@ -5,7 +5,9 @@ VERSION := git-20170908
 
 include config.mk
 
-CPPFLAGS += -DVERSION=\"$(VERSION)\" -DHAVE_GIFLIB=$(HAVE_GIFLIB) -DHAVE_LIBEXIF=$(HAVE_LIBEXIF)
+VPATH := $(SRCDIR)
+
+CPPFLAGS += -I. -DVERSION=\"$(VERSION)\" -DHAVE_GIFLIB=$(HAVE_GIFLIB) -DHAVE_LIBEXIF=$(HAVE_LIBEXIF)
 DEPFLAGS := -MMD -MP
 
 LDLIBS := -lImlib2 -lX11 -lXft
@@ -31,7 +33,7 @@ $(OBJ): Makefile
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(DEPFLAGS) -c -o $@ $<
 
 config.h:
-	cp config.def.h $@
+	cp $(SRCDIR)/config.def.h $@
 
 sxiv:	$(OBJ)
 	$(CC) $(LDFLAGS) -o $@ $(OBJ) $(LDLIBS)

config.mk

@@ -1,3 +1,5 @@
+SRCDIR    := .
+
 PREFIX    := /usr/local
 MANPREFIX := $(PREFIX)/share/man