configure: added --enable-rpath option.

If set, binaries will have rpath set to $ORIGIN/../lib. That means
that even using defaults (--prefix=/usr), you can install
it via DESTDIR anywhere in your system and binaries will work.

This is in preparation for BSD support.

Closes: #87 [via git-merge-pr]

mk/prog.mk

@@ -49,7 +49,7 @@ endif
 $(BIN).static: $(OBJS) $(STATIC_LIBS)
 	@printf " [CCLD]\t\t$@\n"
 	${SILENT}$(CC) -static $(OBJS) $(CPPFLAGS) -L$(TOPDIR)/lib \
-		$(CFLAGS) $(LDFLAGS) $(STATIC_LIBS) -o $@
+		$(CFLAGS) $(LDFLAGS) $(PROG_LDFLAGS) $(STATIC_LIBS) -o $@
 
 $(BIN): $(OBJS)
 	@printf " [CCLD]\t\t$@\n"

mk/test.mk

@@ -36,5 +36,5 @@ uninstall:
 $(TEST): $(OBJS)
 	@printf " [CCLD]\t\t$@\n"
 	${SILENT}$(CC) $^ $(CPPFLAGS) -L$(TOPDIR)/lib $(CFLAGS) \
-		$(PROG_CFLAGS) $(LDFLAGS) -lxbps -latf-c -o $@
+		$(PROG_CFLAGS) $(LDFLAGS) $(PROG_LDFLAGS) -lxbps -latf-c -o $@