unused variables and other nits in gnulib/m4

[Ralf Wildenhues]

Hello Simon, Jim, Paul, Bruno, all,

OK to apply these patches to make
  ./configure CC='gcc -Wall -Werror -fno-builtin'

work better (tested on GNU/Linux) with these tests?
(Mind you, I haven't tested any of these changes on other systems.)

Bruno, maybe it's time to kill signed.m4 or make it a noop?

Cheers,
Ralf

        * fnmatch.m4 (_AC_FUNC_FNMATCH_IF): Avoid expression that is
        always true.
        * strndup.m4 (gl_FUNC_STRNDUP): include stdlib.h, for `free'.

        * sockpfaf.m4 (gl_SOCKET_FAMILIES): Avoid gcc -Wall warnings.

        * fsusage.m4 (gl_STATFS_TRUNCATES): Avoid unused variable.
        * getdate.m4 (gl_C_COMPOUND_LITERALS): Likewise.
        * jm-winsz1.m4 (gl_HEADER_TIOCGWINSZ_IN_TERMIOS_H): Likewise;
        also add missing comma that caused broken test.
        * link-follow.m4 (gl_AC_FUNC_LINK_FOLLOWS_SYMLINK): Include
        stdlib.h, for `abort'.
        * ls-mntd-fs.m4 (gl_LIST_MOUNTED_FILE_SYSTEMS): Avoid unused
        variables.
        * mkdir-slash.m4 (gl_FUNC_MKDIR_TRAILING_SLASH): Include
        unistd.h if present, for `rmdir'.
        * physmem.m4 (gl_SYS__SYSTEM_CONFIGURATION): Avoid unused
        variables.
        * putenv.m4 (gl_FUNC_PUTENV): Rewrite using AC_RUN_IFELSE, and
        in the process include standard headers for prototypes.
        * readutmp.m4 (gl_READUTMP): Require AC_GNU_SOURCE, so utmpxname
        gets declared on GNU/Linux.
        * rmdir-errno.m4 (gl_FUNC_RMDIR_NOTEMPTY): Include unistd.h, for
        `rmdir'.
        * time_r.m4 (gl_TIME_R): Avoid unused variables.

        * lock.m4 (gl_LOCK_BODY): Avoid unused variables.
        * mbswidth.m4 (gl_MBSWIDTH): Likewise.
        * signed.m4 (bh_C_SIGNED): Likewise.
        * stpncpy.m4 (gl_FUNC_STPNCPY): Likewise.
        * vasprintf.m4 (gl_FUNC_VASPRINTF): Require AC_GNU_SOURCE, so
        vasprintf is declared on GNU/Linux.

all.diff
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simon.diff
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jimpaul.diff
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bruno.diff
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