[CMake] Building Boost as an ExternalProject

[Knox, Kent]

Hi Michael~

> That does seem to be the rub, at least on a Windows build. Something about ExternalProject_Add ain't quite kosher with what the Boost build system expects and it needs a little help.

> Using ExternalProject_Add, it succeeds, to a point. Then it falls over looking for b2 no matter how I try to persuade it:

> ExternalProject_Add (boost
>    PREFIX ${CMAKE_CURRENT_BINARY_DIR}/boost
>    GIT_REPOSITORY git at github.com:boostorg/boost.git
>    GIT_TAG boost-${BOOST_VERSION}
>    GIT_PROGRESS 1
>    UPDATE_COMMAND ${BOOST_BOOTSTRAP_CMD}
>    CONFIGURE_COMMAND ""
>    BUILD_COMMAND ${BOOST_BUILD_CMD} stage
>    INSTALL_COMMAND ""
>    LOG_DOWNLOAD 1
>    LOG_UPDATE 1
>    LOG_CONFIGURE 1
>    LOG_BUILD 1
>    LOG_INSTALL 1
> )

I think you need to add the following to your ExternalProject_Add ()
BUILD_IN_SOURCE  1