Error when running 'gem install watir'

WatirMakeError

C:\Ruby\devkit>gem install watir
Fetching: win32-api-1.4.8-x86-mingw32.gem (100%)
Fetching: windows-api-0.4.0.gem (100%)
Fetching: windows-pr-1.2.1.gem (100%)
Fetching: win32-process-0.6.5.gem (100%)
Fetching: xml-simple-1.1.1.gem (100%)
Fetching: rake-0.9.2.2.gem (100%)
Fetching: hoe-2.12.3.gem (100%)
Fetching: s4t-utils-1.0.4.gem (100%)
WARNING: s4t-utils-1.0.4 has an invalid nil value for @cert_chain
Fetching: builder-3.0.0.gem (100%)
Fetching: user-choices-1.1.6.1.gem (100%)
Fetching: commonwatir-2.0.3.gem (100%)
Fetching: nokogiri-1.5.0-x86-mingw32.gem (100%)
Fetching: ffi-1.0.10.gem (100%)
Temporarily enhancing PATH to include DevKit...
Building native extensions.  This could take a while...Building native extensions.  This could take a while...
ERROR:  Error installing watir:
        ERROR: Failed to build gem native extension.

        C:/Ruby/bin/ruby.exe extconf.rb
checking for ffi.h... no
checking for ffi.h in /usr/local/include... no
checking for rb_thread_blocking_region()... no
checking for ruby_thread_has_gvl_p()... no
checking for ruby_native_thread_p()... no
checking for rb_thread_call_with_gvl()... no
creating extconf.h
creating Makefile

make
gcc -I. -I. -IC:/Ruby/lib/ruby/1.8/i386-mingw32 -I. -DRUBY_EXTCONF_H=\"extconf.h
\"    -g -O2 -DFD_SETSIZE=256   -Wno-declaration-after-statement   -c AbstractMe
mory.c
In file included from AbstractMemory.c:28:0:
compat.h:65:20: warning: extra tokens at end of #ifndef directive
compat.h:69:24: warning: extra tokens at end of #ifndef directive
In file included from AbstractMemory.h:29:0,
                 from AbstractMemory.c:29:
Types.h:68:17: fatal error: ffi.h: No such file or directory
compilation terminated.
make: *** [AbstractMemory.o] Error 1

Gem files will remain installed in C:/Ruby/lib/ruby/gems/1.8/gems/ffi-1.0.10 for
 inspection.
Results logged to C:/Ruby/lib/ruby/gems/1.8/gems/ffi-1.0.10/ext/ffi_c/gem_make.o
ut