getopt-gnu: avoid crash in glibc getopt

getopt-gnu: avoid crash in glibc getopt

Use of "W;" in an option string may be rare, but we might as well
avoid crashing on it.

* m4/getopt.m4 (gl_GETOPT_CHECK_HEADRS): Detect the problem.
* tests/test-getopt.h (test_getopt): Enhance test.
* tests/test-getopt_long.h (test_getopt_long): Likewise.
* doc/posix-functions/getopt.texi (getopt): Document it.
* doc/glibc-functions/getopt_long.texi (getopt_long): Likewise.
* doc/glibc-functions/getopt_long_only.texi (getopt_long_only):
Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>