BUILD/MEDIUM: force a full rebuild if some build options change

We now instrument the makefile to keep a copy of previous build options.
The goal is to ensure that we'll rebuild everything when build options
change. The options that are watched are TARGET, VERBOSE_CFLAGS, and
BUILD_OPTIONS. These ones are copied into a file ".build_opts" and
compared to the new ones upon each build. This file is referenced in
the DEP variable which all .o files depend on, and it depends on the
code which updates it only upon changes. This ensures that a new file
is regenerated and detected upon change and that everything is rebuilt.

Makefile

@@ -770,7 +770,10 @@ LIB_EBTREE = $(EBTREE_DIR)/libebtree.a
 
 # Used only for forced dependency checking. May be cleared during development.
 INCLUDES = $(wildcard include/*/*.h ebtree/*.h)
-DEP = $(INCLUDES)
+DEP = $(INCLUDES) .build_opts
+
+# Used only to force a rebuild if some build options change
+.build_opts: $(shell rm -f .build_opts.new; echo \'$(TARGET) $(BUILD_OPTIONS) $(VERBOSE_CFLAGS)\' > .build_opts.new; if cmp -s .build_opts .build_opts.new; then rm -f .build_opts.new; else mv -f .build_opts.new .build_opts; fi)
 
 haproxy: $(OBJS) $(OPTIONS_OBJS) $(EBTREE_OBJS)
 	$(LD) $(LDFLAGS) -o $@ $^ $(LDOPTS)
@@ -800,12 +803,12 @@ src/haproxy.o:	src/haproxy.c $(DEP)
 	      -DBUILD_OPTIONS='"$(strip $(BUILD_OPTIONS))"' \
 	       -c -o $@ $<
 
-src/haproxy-systemd-wrapper.o:	src/haproxy-systemd-wrapper.c
+src/haproxy-systemd-wrapper.o:	src/haproxy-systemd-wrapper.c $(DEP)
 	$(CC) $(COPTS) \
 	      -DSBINDIR='"$(strip $(SBINDIR))"' \
 	       -c -o $@ $<
 
-src/dlmalloc.o: $(DLMALLOC_SRC)
+src/dlmalloc.o: $(DLMALLOC_SRC) $(DEP)
 	$(CC) $(COPTS) -DDEFAULT_MMAP_THRESHOLD=$(DLMALLOC_THRES) -c -o $@ $<
 
 install-man:
@@ -837,7 +840,7 @@ uninstall:
 	rm -f "$(DESTDIR)$(SBINDIR)"/haproxy-systemd-wrapper
 
 clean:
-	rm -f *.[oas] src/*.[oas] ebtree/*.[oas] haproxy test
+	rm -f *.[oas] src/*.[oas] ebtree/*.[oas] haproxy test .build_opts .build_opts.new
 	for dir in . src include/* doc ebtree; do rm -f $$dir/*~ $$dir/*.rej $$dir/core; done
 	rm -f haproxy-$(VERSION).tar.gz haproxy-$(VERSION)$(SUBVERS).tar.gz
 	rm -f haproxy-$(VERSION) haproxy-$(VERSION)$(SUBVERS) nohup.out gmon.out