Fix installaton of python wrapper

Fix installaton of python wrapper

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The python wrapper installation path is not correct:
-- Building python bindings via swig. Will be installed under /usr/site-packages

In fact it doesn't even rely on CMAKE_INSTALL_PREFIX because of this:
GET_FILENAME_COMPONENT(SITEPACKAGE ${PYTHON_LIB_INSTALL}/../site-packages ABSOLUTE)

I think it should more look like this: (here with CMAKE_INSTALL_PREFIX=/home/michel/Download/libftdi-git/build/install)
-- Installing: /home/michel/Download/libftdi-git/build/install/lib64/python2.7/site-packages/_ftdi.so
-- Installing: /home/michel/Download/libftdi-git/build/install/lib64/python2.7/site-packages/ftdi.py
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