more clear error message

[wchristian]

When encountering a message about a missing library in Makefile output, it can be very unclear what's happening:

https://gist.github.com/09065609fcd0847ce96d

This commit changes the message to explain a bit better what it's about.

[mattn]

I merged it. Thanks.

[wchristian]

Awesome, thanks a lot. :)