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 f72ae8e..de51e58 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -89,9 +89,9 @@
 # tls
 libmbedtls.a: $(OBJS_TLS)
 	echo "  AR    $@"
-	$(AR) rc $@ $(OBJS_TLS)
+	$(AR) -rc $@ $(OBJS_TLS)
 	echo "  RL    $@"
-	$(AR) s $@
+	$(AR) -s $@
 
 libmbedtls.$(SOEXT_TLS): $(OBJS_TLS) libmbedx509.so
 	echo "  LD    $@"
@@ -112,9 +112,9 @@
 # x509
 libmbedx509.a: $(OBJS_X509)
 	echo "  AR    $@"
-	$(AR) rc $@ $(OBJS_X509)
+	$(AR) -rc $@ $(OBJS_X509)
 	echo "  RL    $@"
-	$(AR) s $@
+	$(AR) -s $@
 
 libmbedx509.$(SOEXT_X509): $(OBJS_X509) libmbedcrypto.so
 	echo "  LD    $@"
@@ -135,9 +135,9 @@
 # crypto
 libmbedcrypto.a: $(OBJS_CRYPTO)
 	echo "  AR    $@"
-	$(AR) rc $@ $(OBJS_CRYPTO)
+	$(AR) -rc $@ $(OBJS_CRYPTO)
 	echo "  RL    $@"
-	$(AR) s $@
+	$(AR) -s $@
 
 libmbedcrypto.$(SOEXT_CRYPTO): $(OBJS_CRYPTO)
 	echo "  LD    $@"