win32: fix the search path using HKEYs in cmake

When using the instaler, the search path used for vle has to rely
default HKEY value.
cmake: fix the error message in gvle

the message was the same as in vle.
win32: fix the predefined variables for Boost

In SpawnWin, boost variables that ar used to find boost were
not defined as 8.3 path
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.