* include/bits/std_limits.h(__glibcpp_char_is_signed): Use
       __CHAR_UNSIGNED__.

       * config/os/gnu-linux/bits/os_defines.h(__glibcpp_char_is_signed):
       Likewise.
       * config/os/aix/bits/os_defines.h(__glibcpp_plain_char_is_signed):
       Remove.

From-SVN: r44791