C:\Dropbox\Apps\rails_projects\cardbinder>gem install bson -v '2.1.0'
Temporarily enhancing PATH to include DevKit...
Building native extensions. This could take a while...
ERROR: Error installing bson:
ERROR: Failed to build gem native extension.

C:/RailsInstaller/Ruby2.0.0/bin/ruby.exe extconf.rb
creating Makefile

make "DESTDIR="
generating native-i386-mingw32.def
compiling native.c
In file included from c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby/defines.h:153:0,
from c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby/ruby.h:70,
from c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby.h:33,
from native.c:26:
c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby/win32.h: In function 'rb_w32_pow':
c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby/win32.h:787:5: warning: implicit declaration of function '_controlfp' [-Wimplicit-function-declaration]
c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby/win32.h:788:16: error: '_PC_64' undeclared (first use in this function)
c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby/win32.h:788:16: note: each undeclared identifier is reported only once for each function it appears in
c:/RailsInstaller/Ruby2.0.0/include/ruby-2.0.0/ruby/win32.h:788:24: error: '_MCW_PC' undeclared (first use in this function)
make: * [native.o] Error 1

In my gemfile:

source 'https://rubygems.org'
gem 'rails', '4.0.1'
gem 'activesupport', '4.0.1'
gem 'mongoid', github: 'mongoid/mongoid'

Just a thought, but if I don't add the github tag (or a version number) it installs mongoid v1.x.x. Seems odd.

Any thoughts on what's going on with this?