title: gem install json fails with redifinition of struct timezone/timespec
body: <p>I am on Windows with Ruby 1.9.3 with DevKit (all 32 bit on Win 7 64 bit). Now I try to install rails, but get an error from bundle. If I try to run (what bundle is complaining about)</p>
  
  <pre><code> gem install json
  </code></pre>
  
  <p>I get the following error message:</p>
  
  <pre><code>D:\RubyTest&gt;gem install json
  Temporarily enhancing PATH to include DevKit...
  Building native extensions.  This could take a while...
  ERROR:  Error installing json:
          ERROR: Failed to build gem native extension.
  
          D:/Ruby193/bin/ruby.exe extconf.rb
  creating Makefile
  
  make
  generating generator-i386-mingw32.def
  compiling generator.c
  In file included from d:/Ruby193/include/ruby-1.9.1/ruby/ruby.h:1381:0,
                   from d:/Ruby193/include/ruby-1.9.1/ruby.h:32,
                   from ../fbuffer/fbuffer.h:5,
                   from generator.c:1:
  d:/Ruby193/include/ruby-1.9.1/ruby/missing.h:41:8: error: redefinition of 'struc
  t timespec'
  In file included from d:\ruby193\mingw\bin\../lib/gcc/i686-w64-mingw32/4.7.2/../
  ../../../i686-w64-mingw32/include/process.h:12:0,
                   from d:/Ruby193/include/ruby-1.9.1/ruby/win32.h:62,
                   from d:/Ruby193/include/ruby-1.9.1/ruby/defines.h:223,
                   from d:/Ruby193/include/ruby-1.9.1/ruby/ruby.h:67,
                   from d:/Ruby193/include/ruby-1.9.1/ruby.h:32,
                   from ../fbuffer/fbuffer.h:5,
                   from generator.c:1:
  d:\ruby193\mingw\bin\../lib/gcc/i686-w64-mingw32/4.7.2/../../../../i686-w64-ming
  w32/include/sys/types.h:89:8: note: originally defined here
  In file included from d:/Ruby193/include/ruby-1.9.1/ruby/ruby.h:1381:0,
                   from d:/Ruby193/include/ruby-1.9.1/ruby.h:32,
                   from ../fbuffer/fbuffer.h:5,
                   from generator.c:1:
  d:/Ruby193/include/ruby-1.9.1/ruby/missing.h:48:8: error: redefinition of 'struc
  t timezone'
  In file included from d:/Ruby193/include/ruby-1.9.1/ruby/win32.h:63:0,
                   from d:/Ruby193/include/ruby-1.9.1/ruby/defines.h:223,
                   from d:/Ruby193/include/ruby-1.9.1/ruby/ruby.h:67,
                   from d:/Ruby193/include/ruby-1.9.1/ruby.h:32,
                   from ../fbuffer/fbuffer.h:5,
                   from generator.c:1:
  d:\ruby193\mingw\bin\../lib/gcc/i686-w64-mingw32/4.7.2/../../../../i686-w64-ming
  w32/include/time.h:260:8: note: originally defined here
  make: *** [generator.o] Error 1
  
  
  Gem files will remain installed in D:/Ruby193/lib/ruby/gems/1.9.1/gems/json-1.7.
  7 for inspection.
  Results logged to D:/Ruby193/lib/ruby/gems/1.9.1/gems/json-1.7.7/ext/json/ext/ge
  nerator/gem_make.out
  </code></pre>
  
  <p>Any idea?</p>