On OSX Mavericks, using the latest Xcode (5.5) I’ve seen this error. I tried a few solutions around the net, but only one worked for me.
The solution that worked for me was installing the gems that throw this error as:
ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future gem install <gemName>
For me, this solved my problems with Cucumber and Watir-Webdriver gems.
Other people had success with
sudo xcode-select --install
This didn’t work for me at all.