What Pololu product are using? (A link to the product's page would be great.)
If you are trying to compile that library with one of the Pololu AVR C/C++ Library example templates, can you look at your Atmel Studio .cproj file using a text editor program (e.g. notepad) to see if there is a tag like
<Compile Include="*.c"> in it? If so, that tag could cause trouble when new .c files are added to the project. I recommend changing "*.c" to list the full name of a file or remove that tag.