[Xquartz-dev] X11 development headers

Hi, I'm having lots of problems when compiling various packages with macports, and they all come down to one issue: the X11 development headers seem to be missing. I'm running 10.5.4 (upgraded from 10.5.2) with first Apple's X11 installed, then later upgraded to Xquartz 2.3.1rc1. I also have Xcode 3.1 installed (upgraded from 3.0). This should be enough for everything to work, right? How can I check wether the headers are really missing, i.e. where should I look for them? regards, Joachim