Hmmm, that's a weird one! My only explanation is that you have the file /usr/lib/pkgconfig/lv2-plugin.pc but not /usr/lib/pkgconfig/lv2-gui.pc... Could you check on your system? Btw, what distribution do you use? From my side, I'm not even sure I need to check for both libraries in my wscript... As you said, they come from the same lv2-c++-tools package.