nokogiri error 2

Steves-MacBook-Air:dock steve$ gem install nokogiri -v '1.9.1' --source 'https://rubygems.org/'
Building native extensions. This could take a while...
ERROR:  Error installing nokogiri:
        ERROR: Failed to build gem native extension.

    current directory: /Users/steve/.rbenv/versions/2.6.2/lib/ruby/gems/2.6.0/gems/nokogiri-1.9.1/ext/nokogiri
/Users/steve/.rbenv/versions/2.6.2/bin/ruby -I /Users/steve/.rbenv/versions/2.6.2/lib/ruby/2.6.0 -r ./siteconf20190506-31762-hxs46y.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=/Users/steve/.rbenv/versions/2.6.2/bin/$(RUBY_BASE_NAME)
        --help
        --clean
/Users/steve/.rbenv/versions/2.6.2/lib/ruby/2.6.0/mkmf.rb:467:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
        from /Users/steve/.rbenv/versions/2.6.2/lib/ruby/2.6.0/mkmf.rb:585:in `block in try_compile'
        from /Users/steve/.rbenv/versions/2.6.2/lib/ruby/2.6.0/mkmf.rb:532:in `with_werror'
        from /Users/steve/.rbenv/versions/2.6.2/lib/ruby/2.6.0/mkmf.rb:585:in `try_compile'
        from extconf.rb:138:in `nokogiri_try_compile'
        from extconf.rb:162:in `block in add_cflags'
        from /Users/steve/.rbenv/versions/2.6.2/lib/ruby/2.6.0/mkmf.rb:643:in `with_cflags'
        from extconf.rb:161:in `add_cflags'
        from extconf.rb:412:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/steve/.rbenv/versions/2.6.2/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-18/2.6.0-static/nokogiri-1.9.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/steve/.rbenv/versions/2.6.2/lib/ruby/gems/2.6.0/gems/nokogiri-1.9.1 for inspection.