Failed to build gem native extension

[Dominique __]

When trying to install nokigiri I get the following error

Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
        ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.1 -r ./siteconf20151019-10253-7f7jr4.rb 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=/usr/bin/ruby2.1
        --help
        --clean
/usr/lib/ruby/2.1.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
        from /usr/lib/ruby/2.1.0/mkmf.rb:571:in `block in try_compile'
        from /usr/lib/ruby/2.1.0/mkmf.rb:522:in `with_werror'
        from /usr/lib/ruby/2.1.0/mkmf.rb:571:in `try_compile'
        from extconf.rb:80:in `nokogiri_try_compile'
        from extconf.rb:87:in `block in add_cflags'
        from /usr/lib/ruby/2.1.0/mkmf.rb:621:in `with_cflags'
        from extconf.rb:86:in `add_cflags'
        from extconf.rb:337:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.1.0/gems/nokogiri-1.6.6.2 for inspection.
Results logged to /var/lib/gems/2.1.0/extensions/x86_64-linux/2.1.0/nokogiri-1.6.6.2/gem_make.out

I already installed:
ruby gems 2.4.8
ruby 2.1.5p273

ruby-dev 
zlib1g-dev

libgmp-dev
libxslt-dev
libxml2-dev

What am I doing wrong here?