xxx@onyx ~ $ emerge --update --verbose-conflicts dev-libs/boost dev-util/boost-build  

These are the packages that would be merged, in order:

Calculating dependencies... done! 
[ebuild     U  ] dev-util/boost-build-1.63.0::gentoo [1.62.0-r1::gentoo] USE="-examples -python {-test}" PYTHON_TARGETS="python2_7" 0 KiB 
[ebuild     U  ] dev-libs/boost-1.63.0:0/1.63.0::gentoo [1.62.0-r1:0/1.62.0::gentoo] USE="context icu nls python threads -debug -doc -mpi -static-libs -tools" ABI_X86="(64) -32 (-x32)" PYTHON_TARGETS="python2_7 python3_4 -python3_5" 0 KiB  

Total: 2 packages (2 upgrades), Size of downloads: 0 KiB  

!!! Multiple package instances within a single package slot have been pulled 
!!! into the dependency graph, resulting in a slot conflict:  

dev-libs/boost:0

  (dev-libs/boost-1.63.0:0/1.63.0::gentoo, ebuild scheduled for merge) pulled in by
    dev-libs/boost (Argument)

  (dev-libs/boost-1.62.0-r1:0/1.62.0::gentoo, installed) pulled in by
    dev-libs/boost:0/1.62.0= required by (dev-cpp/libcmis-0.5.2_pre20160820:0.5/0.5::gentoo, installed)
                  ^^^^^^^^^^     
    >=dev-libs/boost-1.46.0:0/1.62.0= required by (media-video/mkvtoolnix-9.8.0:0/0::gentoo, installed)
                           ^^^^^^^^^^i
    dev-libs/boost:0/1.62.0= required by (dev-util/mdds-0.12.1:0/0.12.1::gentoo, installed)
                  ^^^^^^^^^^
    dev-libs/boost:0/1.62.0=[abi_x86_64(-),threads] required by (net-libs/nghttp2-1.19.0:0/1.14::gentoo, installed)
                  ^^^^^^^^^^
    dev-libs/boost:0/1.62.0= required by (dev-db/mysql-connector-c++-1.1.6:0/0::gentoo, installed)
                  ^^^^^^^^^^
    dev-libs/boost:0/1.62.0= required by (dev-util/mdds-1.2.2:1/1.2::gentoo, installed)
                  ^^^^^^^^^^
    >=dev-libs/boost-1.55:0/1.62.0= required by (app-office/libreoffice-5.2.4.2:0/0::gentoo, installed)
                         ^^^^^^^^^^
    >=dev-libs/boost-1.51.0:0/1.62.0= required by (dev-libs/liborcus-0.11.2:0/0.11::gentoo, installed)
                           ^^^^^^^^^^
    dev-libs/boost:0/1.62.0= required by (dev-libs/librevenge-0.0.4:0/0::gentoo, installed)
                  ^^^^^^^^^^
    dev-libs/boost:0/1.62.0=[threads] required by (dev-libs/libixion-0.11.1:0/0.11::gentoo, installed)
                  ^^^^^^^^^^
    dev-libs/boost:0/1.62.0=[threads] required by (media-sound/mp3diags-1.2.03-r1:0/0::gentoo, installed)
                  ^^^^^^^^^^   


It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are 
impossible to satisfy simultaneously.  If such a conflict exists in 
the dependencies of two different packages, then those packages can 
not be installed simultaneously. You may want to try a larger value of 
the --backtrack option, such as --backtrack=30, in order to see if 
that will solve this conflict automatically.

For more information, see MASKED PACKAGES section in the emerge man 
page or refer to the Gentoo Handbook.