I'm having trouble installing therubyracer gem on Windows.
Using Ruby 2.1.6 32-bit and running 
gem install libv8 -v '3.16.14.3' -- --with-system-v8
This is the error I get:
Installing therubyracer 0.12.1 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
    C:/Ruby21/bin/ruby.exe extconf.rb
checking for main() in -lpthread... no
checking for v8.h... no
*** 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:/Ruby21/bin/ruby
        --with-pthreadlib
        --without-pthreadlib
        --enable-debug
        --disable-debug
        --with-v8-dir
        --without-v8-dir
        --with-v8-include
        --without-v8-include=${v8-dir}/include
        --with-v8-lib
        --without-v8-lib=${v8-dir}/lib
C:/Ruby21/lib/ruby/gems/2.1.0/gems/libv8-3.16.14.3/ext/libv8/location.rb:50:in 
`configure': You have chosen to use the version of V8 found on your system (Libv8::Location::System::NotFoundError)
and *not* the one that is bundle with the libv8 rubygem. However,
it could not be located. please make sure you have a version of
v8 that is compatible with 3.16.14.3 installed. You may
need to special --with-v8-dir options if it is in a non-standard
location
thanks,
The Mgmt
        from C:/Ruby21/lib/ruby/gems/2.1.0/gems/libv8-3.16.14.3/lib/libv8.rb:7:in `configure_makefile'
        from extconf.rb:32:in `<main>'
extconf failed, exit code 1
Gem files will remain installed in C:/Ruby21/lib/ruby/gems/2.1.0/gems/therubyracer-0.12.1 for inspection.
Results logged to C:/Ruby21/lib/ruby/gems/2.1.0/extensions/x86-mingw32/2.1.0/therubyracer-0.12.1/gem_make.out
An error occurred while installing therubyracer (0.12.1), and Bundler cannot continue.
Make sure that `gem install therubyracer -v '0.12.1'` succeeds before bundling.