How do you build the x64 Boost libraries on Windows?

The accepted answer is correct. Adding this in case somebody else googles this answer and still fails to produce x64 version.

Following is what I had to do to build Boost 1.63 on Visual Studio 15 2017 Community Edition.

Commands executed from VS environment cmd shell. Tools -> Visual Studio Command Prompt

C:\Work\Boost_1_63> C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat amd64
C:\Work\Boost_1_63> bootstrap.bat
C:\Work\Boost_1_63> bjam -j4 architecture=x86 address-model=64 link=static stage
C:\Work\Boost_1_63> bjam --prefix=C:\opt\boost architecture=x86 address-model=64 link=static install

You can verify that the resulting .lib is x64 with dumpbin:

C:\Work> dumpbin /headers C:\work\boost_1_63\stage\lib\libboost_locale-vc140-mt-1_63.lib | findstr machine
8664 machine (x64)
8664 machine (x64)
8664 machine (x64)
8664 machine (x64) 
...

You need to add the address-model=64 parameter.

Look e.g. here.