clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]

If you got this error while installing a gem, it's because the compiler changed with release of Xcode 5.1. A quick fix is to change the ARCHFLAGS

For example, installing a gem named babelish:

sudo ARCHFLAGS="-Wno-error=unused-command-line-argument-hard-error-in-future" gem install babelish

Note: found the solution on SO (without quotes), but did not work until I use the double quotes.