Store character vectors in .mat files (-v6 or -v7) as UTF-16 (bug #58368). * ls-mat5.cc (read_mat5_binary_element): Convert UTF-16 and UTF-32 vectors to UTF-8. Take UTF-8 as is. Fall back to replacing non-ASCII characters with "?" for non-convertable strings or ND arrays. (maybe_convert_to_u16): Add function that converts a string vector to UTF-16. (save_mat5_element_length): Get correct length if string is converted. (save_mat5_binary_element): Convert character vectors to UTF-16 to be able to correctly load them in Matlab if they contain non-ASCII characters. * load-save.cc: Add tests for saving and loading different types of strings. * unistr-wrappers.h, unistr-wrappers.cc (octave_u16_to_u8_wrapper, octave_u32_to_u8_wrapper, octave_u8_to_u16_wrapper): New wrappers for gnulib functions. * bootstrap.conf: Add gnulib modules to list.