Failed to build gem native extension - During Rails Installation - ruby-on-rails

Question

I'm trying to install Ruby on Rails, but I keep getting this error message:
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
I don't know what to do, I've tried searching online for answers, such as installing command line tools, gcc, and others I can't remember, but to no effect. I often get errors doing things that others have said are suppose to resolve the problem too.
Here is the entire error message:
ERROR: Error installing rails:
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 ... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers. Check the mkmf.log file for more details. You may
need configuration options.
Provided configuration options:
--with-opt-dir
--without-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
--help
--clean
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:434:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:in `block in try_compile'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:500:in `with_werror'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:in `try_compile'
from extconf.rb:80:in `nokogiri_try_compile'
from extconf.rb:87:in `block in add_cflags'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:599:in `with_cflags'
from extconf.rb:86:in `add_cflags'
from extconf.rb:337:in `<main>'
Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.6.2 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.6.2/ext/nokogiri/gem_make.out