Allow to install scripts in PYTHON_SCRIPTDIR.
Needed for Gentoo multi-python support.

Index: catkin-0.6.11/cmake/templates/python_distutils_install.sh.in
===================================================================
--- catkin-0.6.11.orig/cmake/templates/python_distutils_install.sh.in
+++ catkin-0.6.11/cmake/templates/python_distutils_install.sh.in
@@ -25,4 +25,4 @@ cd "@INSTALL_CMD_WORKING_DIRECTORY@"
     build --build-base "@CMAKE_CURRENT_BINARY_DIR@" \
     install \
     $DESTDIR_ARG \
-    @SETUPTOOLS_ARG_EXTRA@ --prefix="@CMAKE_INSTALL_PREFIX@" --install-scripts="@CMAKE_INSTALL_PREFIX@/@CATKIN_GLOBAL_BIN_DESTINATION@"
+    @SETUPTOOLS_ARG_EXTRA@ --prefix="@CMAKE_INSTALL_PREFIX@" --install-scripts="${PYTHON_SCRIPTDIR:-@CMAKE_INSTALL_PREFIX@/@CATKIN_GLOBAL_BIN_DESTINATION@}"