It builds fine for me; there must be something special about your system.
Comparing your log to mine, I see this in yours but not in mine:
:info:build gconvert.c: In function 'g_iconv':
:info:build gconvert.c:314: warning: passing argument 2 of 'libiconv' from incompatible pointer type
Perhaps there is a problem with your libiconv port.
I've committed a change to pkgconfig to make the log more verbose. Could you please:
sudo port selfupdate
sudo port clean pkgconfig
sudo port install pkgconfig
And attach the new main.log? Maybe that will help identify the problem better.