client: Modified Makefile and CMake configuration.

- Makefile provides env variables BOOST_ROOT and BUILD_CLIENT_TEST via -D option to CMake now (preserves set options regardless of the environment).
- Override BOOST_LIBRARYDIR only if BOOST_ROOT is not set.


git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@2902 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534