[boost] Build breaking changes

Subject: [boost] Build breaking changes
From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2018-03-28 20:42:46

Hello,

I've just noted that building Boost from master stopped working:

[00:16:51] b2.exe -j4 toolset=gcc --layout=system variant=release
architecture=x86 link=static --with-program_options --with-filesystem
--with-system --with-test --with-atomic --with-thread --with-timer
--with-date_time --with-chrono --with-regex --with-random
--with-context stage

[00:16:52] Performing configuration checks
[00:16:52]
[00:16:52]     - default address-model    : 32-bit
[00:16:53]     - default architecture     : x86
[00:16:53]     - symlinks supported       : yes (cached)
[00:16:54]     - C++11 mutex              : yes
[00:16:55]     - has_icu builds           : no
[00:16:55]     - BOOST_COMP_GNUC >= 4.3.0 : yes
[00:16:55]     - C++11 mutex              : no
[00:16:56]     - has_icu builds           : no
[00:16:56]     - BOOST_COMP_GNUC >= 4.3.0 : no
[00:16:56] error: Name clash for '<pstage\lib>libboost_atomic.a'
[00:16:56] error:
[00:16:56] error: Tried to build the target twice, with property sets having
[00:16:56] error: these incompatible properties:
[00:16:56] error:
[00:16:56] error:     -  <address-model>64
[00:16:56] error:     -  <address-model>32
[00:16:56] error:
[00:16:56] error: Please make sure to have consistent requirements for these
[00:16:56] error: properties everywhere in your project, especially for install
[00:16:56] error: targets.
[00:16:56]
[00:16:56] Command exited with code 1

Previously this build command worked fine. Full log available at
https://ci.appveyor.com/project/apolukhin/boost-cookbook/branch/second_edition/job/y9tsbgi430280u7y

Is this an intended breaking change?