If you’ve recently upgraded XCode to 5.1 on your OS X workstation/laptop you may have run into the following error when trying to install or update a gem:

root# gem install shodan
Fetching: json-1.8.1.gem (100%)
Building native extensions. This could take a while...
ERROR: Error installing shodan:
ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
creating Makefile

make "DESTDIR="
compiling generator.c
linking shared-object json/ext/generator.bundle
clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
make: *** [generator.bundle] Error 1

Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/json-1.8.1 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/gems/json-1.8.1/ext/json/ext/generator/gem_make.out

As discussed here the reason for this sudden error is found in the Xcode Release Notes:

The Apple LLVM compiler in Xcode 5.1 treats unrecognized command-line options as errors. This issue has been seen when building both Python native extensions and Ruby Gems, where some invalid compiler options are currently specified.

According to the blog post it seems that the newer version of the llvm compiler shipping with Xcode 5.1 is a little more restrictive when it comes to warnings. Furthermore it says that:

Projects using invalid compiler options will need to be changed to remove those options.
That is, developers should not expect this change to be reverted in the future.

It turns out that the temporary fix, until everyone updates their gems, is to run the following command instead of the usual gem install command:

root# ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future gem install gem_name

For example:

root# ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future gem install shodan
Building native extensions. This could take a while...
Successfully installed json-1.8.1
Fetching: shodan-1.0.0.gem (100%)
Successfully installed shodan-1.0.0
Parsing documentation for json-1.8.1
Installing ri documentation for json-1.8.1
Parsing documentation for shodan-1.0.0
Installing ri documentation for shodan-1.0.0
2 gems installed

There you go. Hope it helps.