C:\Sites>gem install nio4r -v '1.2.1'
Building native extensions.  This could take a while...
ERROR:  Error installing nio4r:
        ERROR: Failed to build gem native extension.

    C:/RailsInstaller/Ruby2.2.0/bin/ruby.exe -r ./siteconf20160720-4992-1ajk6ob.
rb extconf.rb
checking for unistd.h... *** 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=C:/RailsInstaller/Ruby2.2.0/bin/$(RUBY_BASE_NAME)
C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:456:in `try_do': The compiler
 failed to generate an executable file. (RuntimeError)
You have to install development tools first.
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:587:in `try_cpp'

        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:1060:in `block i
n have_header'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:911:in `block in
 checking_for'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:351:in `block (2
 levels) in postpone'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:321:in `open'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:351:in `block in
 postpone'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:321:in `open'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:347:in `postpone
'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:910:in `checking
_for'
        from C:/RailsInstaller/Ruby2.2.0/lib/ruby/2.2.0/mkmf.rb:1059:in `have_he
ader'
        from extconf.rb:3:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in C:/RailsInstaller/Ruby2.2.0/lib/ruby/gems/2.2
.0/gems/nio4r-1.2.1 for inspection.
Results logged to C:/RailsInstaller/Ruby2.2.0/lib/ruby/gems/2.2.0/extensions/x86
-mingw32/2.2.0/nio4r-1.2.1/gem_make.out