you can see my config.log here: http://www.davidemorelli.it/public/config.log.txt
I didn't notice (in config.log) that libquicktime.so.0 could not be found!
so I recompiled libquicktime configuring it with --prefix=/usr and now I can configure gem without errors (it was in /usr/local/lib)...
so it was my fault, i installed libquicktime in a wrong way...
ciao, davide.