I want to build the Boost libs without ICU support in Regex, but have been unable to prevent it from being linked in.

I've run the following commands on a freshly extracted distribution of 1.57:
    ./bootstrap.sh --without-icu --prefix=/home/rob/workspace/deps/boost/1.57.0/linux/x64/
    ./b2 install

The bootstrap stage output includes this, which seems to indicate it's behaving correctly:
    Unicode/ICU support for Boost.Regex?... disabled.

The resulting project-config.jam makes no mention of ICU.
The build log includes:
    - has_icu builds           : yes
    - icu                      : yes

The shared library dependencies for the resulting libboost_regex.so.1.57.0 then include:
        libicuuc.so.50 => /lib64/libicuuc.so.50 (0x00007f74e5e3b000)
        libicui18n.so.50 => /lib64/libicui18n.so.50 (0x00007f74e5a3b000)
        libicudata.so.50 => /lib64/libicudata.so.50 (0x00007f74e4467000)

I know the docs for Boost.Regex specify the bootstrap argument as --disable-icu, however this is rejected as invalid, unlike the argument I provide.

Have I messed up somewhere or is this a bug?
TIA -- rob