I’ve been having trouble compiling Ruby 1.9.3 since updating to Xcode 4.6. After a little Googling, it turns out that the error is with some invalid CFLAGS set in Ruby’s compile scripts.
The flag can be tweaked as follows:
CFLAGS="-Wno-error=shorten-64-to-32" rbenv install 1.9.3-p392
Hopefully future versions of Ruby will address this.