Use env python for running python so that it uses the proper version on Gentoo.
Honour CATKIN_PREFIX_PATH gentoo addition.

Index: roslib/env-hooks/10.ros.sh.em
===================================================================
--- roslib.orig/env-hooks/10.ros.sh.em
+++ roslib/env-hooks/10.ros.sh.em
@@ -1,8 +1,5 @@
 # generated from ros/env-hooks/10.ros.sh.em
 
-# scrub old ROS bin dirs, to avoid accidentally finding the wrong executables
-export PATH="`@(PYTHON_EXECUTABLE) -c \"import os; print(os.pathsep.join([x for x in \\\"$PATH\\\".split(os.pathsep) if not any([d for d in ['cturtle', 'diamondback', 'electric', 'fuerte'] if d in x])]))\"`"
-
 @{
 # This is a build-time environment variable which allows a build engineer to override the expected
 # ROS_DISTRO value for a workspace, for example to deliberately use a newer version of roslib with
@@ -19,7 +16,7 @@ export ROS_DISTRO=@(ROS_DISTRO)
 PYTHON_CODE_BUILD_ROS_PACKAGE_PATH=$(cat <<EOF
 from __future__ import print_function
 import os
-env_name = 'CMAKE_PREFIX_PATH'
+env_name = 'CATKIN_PREFIX_PATH'
 paths = [path for path in os.environ[env_name].split(os.pathsep)] if env_name in os.environ and os.environ[env_name] != '' else []
 workspaces = [path for path in paths if os.path.exists(os.path.join(path, '.catkin'))]
 paths = []
@@ -38,7 +35,7 @@ for workspace in workspaces:
 print(os.pathsep.join(paths))
 EOF
 )
-export ROS_PACKAGE_PATH="`@(PYTHON_EXECUTABLE) -c \"$PYTHON_CODE_BUILD_ROS_PACKAGE_PATH\"`"
+export ROS_PACKAGE_PATH="`/usr/bin/env python -c \"$PYTHON_CODE_BUILD_ROS_PACKAGE_PATH\"`"
 
 @[if DEVELSPACE]@
 # env variables in develspace