Fix bug with make install without tests

closes #232
diff --git a/Makefile b/Makefile
index e1fb3cf..7f03115 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,7 @@
 	$(MAKE) -C tests
 
 ifndef WINDOWS
-install: all
+install: no_test
 	mkdir -p $(DESTDIR)/include/mbedtls
 	cp -r include/mbedtls $(DESTDIR)/include