You have encountered an issue with the order of the libraries specified to the linker. The linker implementation used by gcc/g++ requires them listed in an order which is opposite from what one would expect.
For details, see this
stack overflow discussion, specifically in the "Linking to Static Libraries" section, where it says:
"The linker searches from left to right, and notes unresolved symbols as it go. Only a library resolves the symbol, it takes the object files of that library to resolve the symbol (b.o out of libb.a in this case).
Dependencies of static libraries against each other work the same - the library that needs symbols must be first, then the library that resolves the symbol."
In this case, since the navx_frc_cpp library is dependent upon the wpi library, the navx_frc_cpp library must be listed before the wpi library in the list in the libraries tab.
This order can be changed (in Eclipse) by bringing up the project properties, and then within the properties dialog selecting C/C++ General -> Paths and Symbols in the tree on the left. Next, within the Paths and Symbols pane, click on the libraries tab. You should see in this list the "wpi" and "navx_frc_cpp" libraries listed. Change their order (by using the move up and move down buttons) and rebuild.