Unfortunately, you’re trying Rails at a bit of a bad time. There’s a lot of transition at the moment, as others have mentioned Xcode 4.2 was recently released and has a new compiler, also Ruby 1.9.3 came out.
I’ve been developing Ruby and Rails for some years now, but trying to get up and running again after installing Lion clean this week has been more problematic than in the past.
First, make sure RVM is up to date (as the issue should be fixed):
rvm get head
Then try installing like so:
CC=/usr/bin/gcc-4.2 rvm install 1.9.3 --enable-shared
EDIT:
Note, working through my own issues I believe the first command probably fixed the problem. The second wouldn’t have provided any benefit over:
rvm install 1.9.3
As on my clean install, /usr/bin/gcc-4.2 doesn’t exist.
Also I wanted to add that I ended up compiling the older GCC manually for those situations where the new compiler fails. I followed this very informative blog post.