build system: introduce FEATURE_ASSUME_UNICODE

build system: introduce FEATURE_ASSUME_UNICODE
ls: use it. also do G trick. with UNICODE off:

function                                             old     new   delta
ls_main                                              843     871     +28
show_color                                             1       -      -1
showdirs                                             500     497      -3
my_stat                                              100      97      -3
terminal_width                                         4       -      -4
tabstops                                               4       -      -4
status                                               122     118      -4
static.dotdir                                          4       -      -4
current_time_t                                         4       -      -4
all_fmt                                                4       -      -4
------------------------------------------------------------------------------
(add/remove: 0/6 grow/shrink: 1/3 up/down: 28/-31)             Total: -3 bytes

Also was tested with UNICODE on with glibc.