Make ar invocation more portable

armar doesn't understand the syntax without dash. OTOH, the syntax with dash
is the only one specified by POSIX, and it's accepted by GNU ar, BSD ar (as
bundled with OS X) and armar, so it looks like the most portable syntax.

fixes #386
diff --git a/library/Makefile b/library/Makefile
index c7c5bf0..6a8d9ba 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -93,9 +93,9 @@
 
 libmbedtls.a: $(OBJS)
 	echo "  AR    $@"
-	$(AR) rc $@ $(OBJS)
+	$(AR) -rc $@ $(OBJS)
 	echo "  RL    $@"
-	$(AR) s $@
+	$(AR) -s $@
 
 libpolarssl.$(DLEXT): libmbedtls.$(DLEXT)
 	echo "  LN    $@ -> $?"