I recently upgraded my laptop to Yosemite from Mavericks (which has been a great upgrade) and a couple days ago I was trying to install nomad but I was receiving an odd error.

Scotts-Air:~ swarren$ sudo gem install nomad-cli
[snip]
Building native extensions.  This could take a while...
ERROR:  Error installing nomad-cli:
    ERROR: Failed to build gem native extension.

    /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
checking if the C compiler accepts ... yes
checking if the C compiler accepts -Wno-error=unused-command-line-argument-hard-error-in-future... no
Building nokogiri using packaged libraries.
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The file "/usr/include/iconv.h" is missing in your build environment,
which means you haven't installed Xcode Command Line Tools properly.

I thought this was odd because I installed the Xcode Command Line Tools in Mavericks so I thought maybe all of the packages were uninstalled when I upgraded but gcc and make were both still installed:

Scotts-Air:~ swarren$ gcc
clang: error: no input files

Scotts-Air:~ swarren$ make
make: *** No targets specified and no makefile found.  Stop.

In order to fix this problem, run the following command:

xcode-select --install

This will bring up the following window and you'll need to press "Install".

And then accept the License Agreement.

And then wait for the installation.

After this the header files were found and I was able to install the software!