remove support for some very old platforms

[Bruno Haible]

Platforms that don't have the functions
  memcmp
  memcpy
  memmove
  memset
  memchr
  dup2
  getcwd
were seen in the years 1990..1995, but had already disappeared around 2000.
There's no reason to keep the code for these obsolete platforms now,
20 years later.


2020-07-07  Bruno Haible  <bruno@clisp.org>

        doc: Remove support for some very old platforms.
        * doc/posix-functions/memcmp.texi: Don't mention "older platforms".
        * doc/posix-functions/memcpy.texi: Likewise.
        * doc/posix-functions/memmove.texi: Likewise.
        * doc/posix-functions/memset.texi: Likewise.
        * doc/posix-functions/getcwd.texi: Likewise.

        memchr: Remove support for some very old platforms.
        * m4/memchr-obsolete.m4: Remove file.
        * modules/memchr-obsolete: Remove file.
        * m4/memchr.m4 (gl_FUNC_MEMCHR): Assume module 'memchr-obsolete' is
        absent. Don't define HAVE_MEMCHR.
        * lib/string.in.h (memchr): Assume HAVE_MEMCHR is 1.
        * modules/memchr (Depends-on): Remove memchr-obsolete.
        (configure.ac): Assume HAVE_MEMCHR is 1.
        * m4/string_h.m4 (gl_HEADER_STRING_H_DEFAULTS): Don't initialize
        HAVE_MEMCHR.
        * modules/string (Makefile.am): Don't substitute HAVE_MEMCHR.
        * doc/posix-functions/memchr.texi: Don't mention module
        'memchr-obsolete'.
        * m4/strcasestr.m4 (gl_FUNC_STRCASESTR_SIMPLE): Assume HAVE_MEMCHR is 1.
        * m4/strstr.m4 (gl_FUNC_STRSTR_SIMPLE): Assume HAVE_MEMCHR is 1.
        * modules/strnlen (Depends-on): Remove memchr-obsolete.

        dup2: Remove support for some very old platforms.
        * m4/dup2-obsolete.m4: Remove file.
        * modules/dup2-obsolete: Remove file.
        * m4/dup2.m4 (gl_FUNC_DUP2): Assume module 'dup2-obsolete' is absent.
        Don't define HAVE_DUP2.
        * lib/unistd.in.h (dup2): Assume HAVE_DUP2 is 1.
        * lib/dup2.c: Likewise.
        * modules/dup2 (Depends-on, configure.ac): Likewise.
        (Depends-on): Remove dup2-obsolete.
        * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Don't initialize HAVE_DUP2.
        * modules/unistd (Makefile.am): Don't substitute HAVE_DUP2.
        * doc/posix-functions/dup2.texi: Don't mention module 'dup2-obsolete'.

0001-dup2-Remove-support-for-some-very-old-platforms.patch
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0002-memchr-Remove-support-for-some-very-old-platforms.patch
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0003-doc-Remove-support-for-some-very-old-platforms.patch
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