当我运行bundle install时，它在尝试使用本机扩展安装ffi gem时失败。我收到此错误
Installing ffi 1.8.1 with native extensions
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
C:/Ruby21-x64/bin/ruby.exe extconf.rb
checking for ffi.h... no
checking for ffi.h in /usr/local/include,/usr/include/ffi... no
checking for rb_thread_blocking_region()... yes
checking for rb_thread_call_with_gvl()... yes
checking for rb_thread_call_without_gvl()... yes
checking for ffi_prep_cif_var()... no
creating extconf.h
creating Makefile
make "DESTDIR="
generating ffi_c-x64-mingw32.def
Configuring libffi
configure: WARNING: if you wanted to set the --build type, don't use --host.
If a cross compiler is detected then cross compile mode will be used
configure: WARNING: cache variable lt_cv_path_LD contains a newline
configure: WARNING: cache variable lt_cv_path_LD contains a newline
make -C "/c/Ruby21-x64/lib/ruby/gems/2.1.0/gems/ffi-1.8.1/ext/ffi_c/libffi-x64-mingw32"
make[1]: Entering directory `/c/Ruby21-x64/lib/ruby/gems/2.1.0/gems/ffi-1.8.1/ext/ffi_c/libffi-x64-mingw32'
Makefile:318: *** missing separator. Stop.
make[1]: Leaving directory `/c/Ruby21-x64/lib/ruby/gems/2.1.0/gems/ffi-1.8.1/ext/ffi_c/libffi-x64-mingw32'
make: *** ["/c/Ruby21-x64/lib/ruby/gems/2.1.0/gems/ffi-1.8.1/ext/ffi_c/libffi-x64-mingw32"/.libs/libffi_convenience.a] Error 2
Gem files will remain installed in C:/Ruby21-x64/lib/ruby/gems/2.1.0/gems/ffi-1.8.1 for inspection.
Results logged to C:/Ruby21-x64/lib/ruby/gems/2.1.0/gems/ffi-1.8.1/ext/ffi_c/gem_make.out
An error occurred while installing ffi (1.8.1), and Bundler cannot continue.
Make sure that `gem install ffi -v '1.8.1'` succeeds before bundling.
我在Windows 7上使用ruby 2.1.7和rails 3.2。我找到了不同的解决方案，如更新系统宝石和降级ffi版本，但它们都不适合我。我该如何解决这个问题？