lib: don't rebuild libcrucible unless there is a version change

If we create an identical .version.cc then don't bother keeping it.
This prevents libcrucible from rebuilding if there are no other changes,
which in turn prevents all the binaries from rebuilding unconditionally.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/Makefile

@@ -40,7 +40,7 @@ depends.mk: $(CRUCIBLE_OBJS:%.o=.depends/%.dep)
 
 .version.cc: configure.h Makefile ../makeflags $(CRUCIBLE_OBJS:.o=.cc) ../include/crucible/*.h
 	echo "namespace crucible { const char *VERSION = \"$(TAG)\"; }" > $@.new
-	mv -f $@.new $@
+	if ! cmp "$@.new" "$@"; then mv -fv $@.new $@; fi
 
 include depends.mk