win32: fix the test_package test

Setting the vle home using a boost::filsesytem::path requires to
format the path string with string() function.
cmake: library dirs in FindVLE and FindGVLE

the library dirs can be empty in the case, for example, of an install in /usr/
on linux using pkg-config. These empty variables should not lead to errors in configure
when using FIND_PACKAGE_HANDLE_STANDARD_ARGS.