The UART library needs to be listed in your app’s options.mk file if your app has one. This tells the Wixel SDK Makefile to use the library when linking the app. If that doesn’t help, could you post the options.mk file for your app here?
The way Wixel apps get compiled is determined by the Wixel SDK’s Makefile and Eclipse just runs “make” to use it. The settings you found don’t affect the contents of the Makefile. You can compile apps without Eclipse.