cmake: improved Python and boost_python configuration
cmake: improved Python and boost_python configuration

dagophil/vigra,dstoe/vigra,dstoe/vigra,funkey/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,dstoe/vigra,dagophil/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,funkey/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,dagophil/vigra,funkey/vigra