Provide static strings for gnc_userdata_home and gnc_userconfig_home.
On Windows boost::filesystem::path's string() produces a string
that goes out of scope with the function in which it's called, so
returning its c_str() ptr yields freed memory, usually full of garbage.
This has an interesting side effect in gnc-file.c's check_file_path():
Since the memory is freed, g_path_get_dirname() reuses it after an
iteration or two, writing its result into the same address. The
following strcmp naturally returns 0 because it's comparing two
instances of the same ptr, so check_file_path falsely reports that
the proposed save path is in GNC_USERDATA_DIR.