Amendment to cmake changes for out of source builds

This is a follow on change to getting out of source cmake builds to work. Our builds are structured to do full out of source builds as a result they require the use of CMAKE_CURRENT_BINARY_DIR. For example, we have a repository that acts as a container for a bunch of submodules like this:

    repos/vendor
    repos/vendor/msgpack-c
    repos/vendor/some-other-submodule

Now, we have to cross compile for several different platforms so we can't simply use the simple 'build' subdirectory approach. Instead we create top level build directories as follows:

    repos/vendor/build/msgpack-c/linux
    repos/vendor/build/msgpack-c/freebsd

As a result, we need the flexibility of CMAKE_CURRENT_BINARY_DIR as CMAKE_BINARY_DIR points to the wrong location.