group-member: Declare function in <unistd.h>.

group-member: Declare function in <unistd.h>.

* lib/unistd.in.h (group_member): New declaration.
* lib/group-member.h: Remove file.
* lib/group-member.c: Include <unistd.h> instead of group-member.h.
* tests/test-unistd-c++.cc: Check signature of group_member.
* m4/group-member.m4 (gl_FUNC_GROUP_MEMBER): Require
gl_UNISTD_H_DEFAULTS. Set HAVE_GROUP_MEMBER.
* m4/unistd_h.m4 (gl_UNISTD_H): Check whether group_member is declared.
(gl_UNISTD_H_DEFAULTS): Initialize GNULIB_GROUP_MEMBER,
HAVE_GROUP_MEMBER.
* modules/group-member (Files): Remove lib/group-member.h.
(Depends-on): Add unistd. Specify conditions.
(configure.ac): Invoke gl_UNISTD_MODULE_INDICATOR.
(Include): Change to <unistd.h>.
* modules/unistd (Makefile.am): Substitute GNULIB_GROUP_MEMBER,
HAVE_GROUP_MEMBER.
* NEWS: Mention the change.
* lib/euidaccess.c: Don't include group-member.h.