According to the Eclipse FAQ on adding libraries:
Adding C/C++ External Libraries
How do I add an external library to my C++ project?
Go to Your Project's Properties by right clicking on project's name and selecting
properties. Click on "C/C++ Build". Under Tool Settings Tab, click on Directories.
Click on The Add Button and Select FileSystem. Select the folder with C/C++ libraries.
Apply and then Ok. The new library is included.
The option commented before didn't work when I tried it. I solve it including the
external library I want to add (libwsock32.a in my case) in Project->Properties->Tool
Settings Tab->C++ Linker->Miscellaneous.
14:26, 18 December 2007 (EST): Neither of these worked for me under Linux. I had to open
up the project's properties and go to "C/C++ Build" -> "Settings" and then hit the
"Libraries" item under "GCC C++ Linker" heading in the list on the right. Then, I
entered the libraries as I would in the command line ("pthread" for libpthread, etc...).