Simplify and modernize strerror substitute, partly to fix Solaris 8 bug. * lib/strerror.c: Include <string.h> always, to test interface, and to remove the need for the dummy. Include intprops.h to compute width instead of doing it ourselves and missing a CHAR_BIT declaration, which broke tar 1.19 on Solaris 8. (strerror): Define it to return NULL if there's no system strerror. (rpl_strerror): Use INT_STRLEN_BOUND to compute bound. Omit !HAVE_STRERROR code. We don't need to worry about supporting ancient pre-strerror Unix systems well any more. Saying "unknown system error" is enough. * lib/string.in.h (strerror): Simplify the ifdef to reflect the simpler strerror.c implementation. * m4/strerror.m4 (gl_FUNC_STRERROR_SEPARATE, gl_PREREQ_STDERROR): Simplify the tests to reflect the simpler strerror implementation. * modules/strerror (Depends-on): Add intprops.