PB installation compass

Bonjour,
je suis sous mac osX el capitan.
J'utilise depuis peu sass et grunt pour mon intégration web. c'est grunt qui gère la compilation de mes .scss en .css.

Je souhaite me rajouter la couche compass pour profiter des mixins,etc...

PROBLEME :
je n'arrive pas à installer compass depuis mon terminal
je fais la commande "sudo gem install compass" (j'ai également fait la maj de ruby avant avec "sudo gem update --system")`
il me sort l'erreur suivante :

"
MBP-de-LE:~ macfabien$ 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: /Library/Ruby/Gems/2.0.0/gems/ffi-1.9.10/ext/ffi_c

/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby -r ./siteconf20160106-25711-c93cc3.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
--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=/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
--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}/
--with-libffi-config
--without-libffi-config
--with-pkg-config
--without-pkg-config
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:434:in try_do': The compiler failed to generate an executable file. (RuntimeError) You have to install development tools first. from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:inblock in try_compile'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:502:in with_werror' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:intry_compile'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:1038:in block in have_header' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:889:inblock in checking_for'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:340:in block (2 levels) in postpone' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:310:inopen'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:340:in block in postpone' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:310:inopen'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:336:in postpone' from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:888:inchecking_for'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:1037: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:

/Library/Ruby/Gems/2.0.0/extensions/universal-darwin-15/2.0.0/ffi-1.9.10/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/ffi-1.9.10 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-15/2.0.0/ffi-1.9.10/gem_make.out
"

quelqu'un aurait il une solution ?