ros-melodic/tf2 compile failed

cmake failed が発生し，/var/tmp/portage/ros-melodic/tf2/temp/error.log を確認すると以下のようなものが．

  Unable to find the requested Boost libraries.

  Boost version: 1.70.0

  Boost include path: /usr/include

  Could not find the following Boost libraries:

          boost_signals

  Some (but not all) of the required Boost libraries were found.  You may
  need to install these additional Boost libraries.  Alternatively, set
  BOOST_LIBRARYDIR to the directory containing Boost libraries or BOOST_ROOT
  to the location of Boost.
Call Stack (most recent call first):
  src/cpp/CMakeLists.txt:208 (find_package)

boost_signals がないと， boost 1.69 より新しいバージョンだとboost_signals2を用いるっぽいですね． boost 1.70 を1.65までダウングレードすることで解決しました．

# emerge -av \=boost-1.65.0