What are you trying to use this library with? Most likely, you have an old version of some build tool or application. Have you updated your entire installation MacPorts?
sudo port selfupdate
sudo port -u upgrade outdated
EDIT: based on your additional information, more questions:
-I /sw/include -L /sw/lib, Fink, MacPorts (/opt/local/{include,lib}). , Fink MacPorts. - ? , Fink ?
, 10.5 , , 10.5? 10,6 , 10,5 10,6? , Xcode .
EDIT: , . Leopard , , 10.6 SDK. , , , Xcode, , , , , , gcc, et al. !