$ gem install compass
Fetching: sass-3.4.23.gem (100%)
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.

sudo gem install compass

$ sudo gem install compass
Building native extensions.  This could take a while...
ERROR:  Error installing compass:
    ERROR: Failed to build gem native extension.

    current directory: /usr/local/lib/ruby/gems/2.4.0/gems/ffi-1.9.18/ext/ffi_c
/usr/local/opt/ruby/bin/ruby -r ./siteconf20170407-4681-fc3mfa.rb extconf.rb
checking for ffi.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
    --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/local/Cellar/ruby/2.4.1_1/bin/$(RUBY_BASE_NAME)
    --with-ffi_c-dir
    --without-ffi_c-dir
    --with-ffi_c-include
    --without-ffi_c-include=${ffi_c-dir}/include
    --with-ffi_c-lib
    --without-ffi_c-lib=${ffi_c-dir}/lib
    --with-libffi-config
    --without-libffi-config
    --with-pkg-config
    --without-pkg-config
/usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:457:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:588:in `try_cpp'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:1095:in `block in have_header'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:945:in `block in checking_for'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:351:in `block (2 levels) in postpone'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:321:in `open'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:351:in `block in postpone'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:321:in `open'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:347:in `postpone'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:944:in `checking_for'
    from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/mkmf.rb:1094:in `have_header'
    from extconf.rb:16:in `<main>'

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

  /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0/ffi-1.9.18/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /usr/local/lib/ruby/gems/2.4.0/gems/ffi-1.9.18 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0/ffi-1.9.18/gem_make.out

xcode-select --install