Just had the same problem installing Gnome. Fortunately I had Flex on my SuSE 9.0 distro's disk. Simply installed the missing package, and continued on.
BTW, yes, I'm using Garnome to build the monster. For the most part, it finds and downloads automatically most of the dependencies needed.
Bear in mind that installation and compilation can take several hours... mine is still running after 12+ hours on a PIII 1000mhz machine, 256mb and ccache enabled. YMMV.