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

Index: catkin-0.8.3/cmake/templates/python_distutils_install.sh.in
===================================================================
--- catkin-0.8.3.orig/cmake/templates/python_distutils_install.sh.in
+++ catkin-0.8.3/cmake/templates/python_distutils_install.sh.in
@@ -30,4 +30,4 @@ echo_and_run /usr/bin/env \
     build --build-base "@CMAKE_CURRENT_BINARY_DIR@" \
     install \
     --root="${DESTDIR-/}" \
-    @SETUPTOOLS_ARG_EXTRA@ --prefix="@CMAKE_INSTALL_PREFIX@" --install-scripts="@CMAKE_INSTALL_PREFIX@/@CATKIN_GLOBAL_BIN_DESTINATION@"
+    @SETUPTOOLS_ARG_EXTRA@ --prefix="@CMAKE_INSTALL_PREFIX@" --install-scripts="${PYTHON_SCRIPTDIR:-@CMAKE_INSTALL_PREFIX@/@CATKIN_GLOBAL_BIN_DESTINATION@}"