Much more portable Makefile

The config.mk file is now optional and only needs to be created if one wants to
persistently overwrite default macro values.

Features used in the Makefile that are not yet in the POSIX standard:

  - Advanced macro assignment operators '+=' and '?=' [1]
  - Special target .PHONY [2]
  - Pattern rules [3]; only needed when $srcdir != '.'. For every pattern rule
    there is an inference rule with the same effect. Hopefully, the inference
    rules get picked up by make programs not supporting pattern rules.
  - Silently including multiple possibly not exisiting files [4] [5]

[1] http://austingroupbugs.net/view.php?id=330
[2] http://austingroupbugs.net/view.php?id=523
[3] http://austingroupbugs.net/view.php?id=513
[4] http://austingroupbugs.net/view.php?id=333
[5] http://austingroupbugs.net/view.php?id=518

Makefile

@@ -1,42 +1,72 @@
-# Requires GNU make 3.80 or later
+# Works best with GNU make 3.80 or later
 
-VERSION := git-20171006
+VERSION = git-20171012
 
-all: sxiv
+srcdir = .
 
-include config.mk
+PREFIX = /usr/local
+MANPREFIX = $(PREFIX)/share/man
 
-override CPPFLAGS += -I. -DVERSION=\"$(VERSION)\" -DHAVE_GIFLIB=$(HAVE_GIFLIB) -DHAVE_LIBEXIF=$(HAVE_LIBEXIF)
+CC ?= gcc
+CFLAGS += -std=c99 -Wall -pedantic
+CPPFLAGS += -I/usr/include/freetype2 -D_XOPEN_SOURCE=700
+DEPFLAGS = -MMD -MP
+LDFLAGS += 
 
-LDLIBS := -lImlib2 -lX11 -lXft
+# autoreload backend: inotify/nop
+AUTORELOAD = inotify
 
-ifneq ($(HAVE_GIFLIB),0)
-	LDLIBS += -lgif
-endif
-ifneq ($(HAVE_LIBEXIF),0)
-	LDLIBS += -lexif
-endif
+# enable features requiring giflib (-lgif)
+HAVE_GIFLIB = 1
 
-SRC := autoreload_$(AUTORELOAD).c commands.c image.c main.c options.c thumbs.c util.c window.c
-DEP := $(SRC:.c=.d)
-OBJ := $(SRC:.c=.o)
+# enable features requiring libexif (-lexif)
+HAVE_LIBEXIF = 1
 
-$(OBJ): config.h Makefile
+REQ_CPPFLAGS = -I. -DVERSION=\"$(VERSION)\" \
+  -DHAVE_GIFLIB=$(HAVE_GIFLIB) -DHAVE_LIBEXIF=$(HAVE_LIBEXIF)
+ALL_CPPFLAGS = $(REQ_CPPFLAGS) $(CPPFLAGS)
 
-%.o: %.c
-	@echo "CC $@"
-	$(CC) $(CFLAGS) $(CPPFLAGS) $(DEPFLAGS) -c -o $@ $<
+LIB_EXIF_0 =
+LIB_EXIF_1 = -lexif
+LIB_GIF_0 =
+LIB_GIF_1 = -lgif
+LDLIBS = -lImlib2 -lX11 -lXft \
+  $(LIB_EXIF_$(HAVE_LIBEXIF)) $(LIB_GIF_$(HAVE_GIFLIB))
 
-config.h: | config.def.h
-	@echo "GEN $@"
-	cp $| $@
+-include config.mk
+
+SRCS = autoreload_$(AUTORELOAD).c commands.c image.c main.c options.c \
+  thumbs.c util.c window.c
+OBJS = $(SRCS:.c=.o)
+DEPS = $(SRCS:.c=.d)
+
+all: sxiv
 
-sxiv:	$(OBJ)
+.PHONY: all clean install uninstall
+.SUFFIXES:
+.SUFFIXES: .c .o
+$(V).SILENT:
+
+sxiv:	$(OBJS)
 	@echo "LINK $@"
-	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)
+	$(CC) $(LDFLAGS) -o $@ $(OBJS) $(LDLIBS)
+
+$(OBJS): config.h $(srcdir)/Makefile
+
+.c.o:
+	@echo "CC $@"
+	$(CC) $(ALL_CPPFLAGS) $(DEPFLAGS) $(CFLAGS) -c -o $@ $<
+
+%.o: $(srcdir)/%.c
+	@echo "CC $@"
+	$(CC) $(ALL_CPPFLAGS) $(DEPFLAGS) $(CFLAGS) -c -o $@ $(srcdir)/$(@:.o=.c)
+
+config.h:
+	@echo "GEN $@"
+	cp $(srcdir)/config.def.h $@
 
 clean:
-	rm -f $(OBJ) $(DEP) sxiv
+	rm -f $(OBJS) $(DEPS) sxiv
 
 install: all
 	@echo "INSTALL bin/sxiv"
@@ -45,7 +75,8 @@ install: all
 	chmod 755 $(DESTDIR)$(PREFIX)/bin/sxiv
 	@echo "INSTALL sxiv.1"
 	mkdir -p $(DESTDIR)$(MANPREFIX)/man1
-	sed "s!PREFIX!$(PREFIX)!g; s!VERSION!$(VERSION)!g" sxiv.1 > $(DESTDIR)$(MANPREFIX)/man1/sxiv.1
+	sed "s!PREFIX!$(PREFIX)!g; s!VERSION!$(VERSION)!g" sxiv.1 \
+		>$(DESTDIR)$(MANPREFIX)/man1/sxiv.1
 	chmod 644 $(DESTDIR)$(MANPREFIX)/man1/sxiv.1
 	@echo "INSTALL share/sxiv/"
 	mkdir -p $(DESTDIR)$(PREFIX)/share/sxiv/exec
@@ -60,9 +91,5 @@ uninstall:
 	@echo "REMOVE share/sxiv/"
 	rm -rf $(DESTDIR)$(PREFIX)/share/sxiv
 
-.PHONY: all clean install uninstall
-.SUFFIXES:
-$(V).SILENT:
-
 -include $(DEP)