MacPorts installs packages in a non unix-standard location. This can cause problems when trying to compile other software against these packages because they aren’t found by the configure script.
I experienced that problem when trying to compile SDL_image, which depends on libraries such as libpng. I had to configure with the following flags :
./configure CPPFLAGS="-I/opt/local/include" LDFLAGS="-L/opt/local/lib"
which indicates in which folders the libraries installed with MacPorts were installed.