root@kl:~# gem install cms_scanner
Building native extensions. This could take a while...
ERROR:  Error installing cms_scanner:
	ERROR: Failed to build gem native extension.

    current directory: /var/lib/gems/2.5.0/gems/nokogiri-1.10.1/ext/nokogiri
/usr/bin/ruby2.5 -I /usr/local/lib/site_ruby/2.5.0 -r ./siteconf20190216-6995-uoeuzb.rb extconf.rb
checking if the C compiler accepts ... yes
Building nokogiri using packaged libraries.
Using mini_portile version 2.4.0
checking for gzdopen() in -lz... no
zlib is missing; necessary for building libxml2
*** 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/$(RUBY_BASE_NAME)2.5
	--help
	--clean
	--use-system-libraries
	--enable-static
	--disable-static
	--with-zlib-dir
	--without-zlib-dir
	--with-zlib-include
	--without-zlib-include=${zlib-dir}/include
	--with-zlib-lib
	--without-zlib-lib=${zlib-dir}/lib
	--enable-cross-build
	--disable-cross-build

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

  /var/lib/gems/2.5.0/extensions/x86_64-linux/2.5.0/nokogiri-1.10.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.5.0/gems/nokogiri-1.10.1 for inspection.
Results logged to /var/lib/gems/2.5.0/extensions/x86_64-linux/2.5.0/nokogiri-1.10.1/gem_make.out
root@kl:~#