rviz fails to build because of wrong Python version

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Hello, I'm trying to install indigo from source on an amd64 gentoo machine. The build is failing when trying to build rviz:

  ==> Processing catkin package: 'rviz'
    ==> Building with env: '/opt/ros/indigo/env.sh'
    ==> cmake /opt/ros/ros_catkin_ws/src/rviz -DCATKIN_DEVEL_PREFIX=/opt/ros/ros_catkin_ws/devel_isolated/rviz -DCMAKE_INSTALL_PREFIX=/opt/ros/indigo -DCMAKE_BUILD_TYPE=Release -DSETUPTOOLS_ARG_EXTRA= -DSETUPTOOLS_DEB_LAYOUT=OFF -G Unix Makefiles in '/opt/ros/ros_catkin_ws/build_isolated/rviz'
    -- Boost version: 1.55.0
    -- Found the following Boost libraries:
    --   filesystem
    --   program_options
    --   signals
 ==> Processing catkin package: 'rviz'
==> Building with env: '/opt/ros/indigo/env.sh'
==> cmake /opt/ros/ros_catkin_ws/src/rviz -DCATKIN_DEVEL_PREFIX=/opt/ros/ros_catkin_ws/devel_isolated/rviz -DCMAKE_INSTALL_PREFIX=/opt/ros/indigo -DCMAKE_BUILD_TYPE=Release -DSETUPTOOLS_ARG_EXTRA= -DSETUPTOOLS_DEB_LAYOUT=OFF -G Unix Makefiles in '/opt/ros/ros_catkin_ws/build_isolated/rviz'
-- Boost version: 1.55.0
-- Found the following Boost libraries:
--   filesystem
--   program_options
--   signals
--   system
--   thread
-- Assimp version has unified headers
-- OGRE_PLUGIN_PATH=/usr/lib64/OGRE
-- Using CATKIN_DEVEL_PREFIX: /opt/ros/ros_catkin_ws/devel_isolated/rviz
-- Using CMAKE_PREFIX_PATH: /opt/ros/indigo
-- This workspace overlays: /opt/ros/indigo
-- Using PYTHON_EXECUTABLE: /usr/bin/python3.4
-- Using default Python package layout
-- Could NOT find PY_em (missing:  PY_EM) 
CMake Error at /opt/ros/indigo/share/catkin/cmake/empy.cmake:29 (message):
  Unable to find either executable 'empy' or Python module 'em'...  try
  installing the package 'python-empy'
Call Stack (most recent call first):
  /opt/ros/indigo/share/catkin/cmake/all.cmake:147 (include)
  /opt/ros/indigo/share/catkin/cmake/catkinConfig.cmake:20 (include)
  CMakeLists.txt:91 (find_package)
    --   system
    --   thread
    -- Assimp version has unified headers
    -- OGRE_PLUGIN_PATH=/usr/lib64/OGRE
    -- Using CATKIN_DEVEL_PREFIX: /opt/ros/ros_catkin_ws/devel_isolated/rviz
    -- Using CMAKE_PREFIX_PATH: /opt/ros/indigo
    -- This workspace overlays: /opt/ros/indigo
    -- Using PYTHON_EXECUTABLE: /usr/bin/python3.4
    -- Using default Python package layout
    -- Could NOT find PY_em (missing:  PY_EM) 
    CMake Error at /opt/ros/indigo/share/catkin/cmake/empy.cmake:29 (message):
      Unable to find either executable 'empy' or Python module 'em'...  try
      installing the package 'python-empy'
    Call Stack (most recent call first):
      /opt/ros/indigo/share/catkin/cmake/all.cmake:147 (include)
      /opt/ros/indigo/share/catkin/cmake/catkinConfig.cmake:20 (include)
      CMakeLists.txt:91 (find_package)

The issue I think is that I do have python-empy installed, but only for python2.7. My understanding is that this was the version I wanted to use for ROS anyway, and all the previous packages seemed to find it. Is there a way to find out why it's trying to use python3.4 or more importantly to tell it to use 2.7?

For what it's worth 2.7 is the system default:

maya ~ # eselect python list
Available Python interpreters:
  [1]   python2.7 *
  [2]   python3.3
  [3]   python3.4
maya ~ # python
Python 2.7.9 (default, Dec 25 2014, 06:57:13) 
[GCC 4.8.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Many thanks, Justin

Update: Following William's suggestion, I ran:

./src/catkin/bin/catkin_make_isolated --install --install-space ${ROS_INSTALL_PREFIX}/indigo -DCMAKE_BUILD_TYPE=Release -DSETUPTOOLS_ARG_EXTRA="" -DSETUPTOOLS_DEB_LAYOUT=OFF -DPYTHON_INCLUDE_DIR=/usr/include/python2.7 -DPYTHON_LIBRARY=/usr/lib64/libpython2.7.so -DPYTHON_EXECUTABLE=/usr/bin/python

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