How do I add a non-standard library in C++ so that I can just add it like a standard library without having to link with a -I option in my makefile?

The problem occurs because I am developing code on multiple servers and with different absolute path directories. I am using the Eigen C++ library and can get it working while calling up the following command in the makefile "-I /usr/local/include/eigen/ -c norm.cpp constants.cpp". The problem occurs when I transfer the server the absolute path is not the same an I have to change it each time I compile. I know I could give it an absolute path but since I use this library a lot I don't want to create strange relative path names.

I am using gcc and have tried placing the eigen folder in /usr/include/c++/4.5, /usr/include/c++/4.5.2 and /usr/local/include/ but it still doesn't work. I am currently using Ubuntu 11.04. 

How do I find the standard library that gcc searches in and how do I add a non-standard library so I can include it like "#include ".