lib: Add getopt-cdefs.h for getopt_long update.

This was modified slightly from Gnulib. In Gnulib, it expects the
@HAVE_SYS_CDEFS_H@ to be replaced. Instead, we can set HAVE_SYS_CDEFS_H
on systems that have it and avoid copying another file into the build
directory. Since we are not using gnulib-tool, copying extra files
requires extra build system updates (and special handling with CMake) so
we should avoid when possible.

lib/getopt-cdefs.h

@@ -0,0 +1,70 @@
+/* getopt-on-non-glibc compatibility macros.
+   Copyright (C) 1989-2023 Free Software Foundation, Inc.
+   This file is part of gnulib.
+   Unlike most of the getopt implementation, it is NOT shared
+   with the GNU C Library.
+
+   This file is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   This file is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#ifndef _GETOPT_CDEFS_H
+#define _GETOPT_CDEFS_H 1
+
+/* This header should not be used directly; include getopt.h or
+   unistd.h instead.  It does not have a protective #error, because
+   the guard macro for getopt.h in gnulib is not fixed.  */
+
+/* getopt-core.h and getopt-ext.h are shared with GNU libc, and expect
+   a number of the internal macros supplied to GNU libc's headers by
+   sys/cdefs.h.  Provide fallback definitions for all of them.  */
+#ifdef HAVE_SYS_CDEFS_H
+# include <sys/cdefs.h>
+#endif
+
+#ifndef __BEGIN_DECLS
+# ifdef __cplusplus
+#  define __BEGIN_DECLS extern "C" {
+# else
+#  define __BEGIN_DECLS /* nothing */
+# endif
+#endif
+#ifndef __END_DECLS
+# ifdef __cplusplus
+#  define __END_DECLS }
+# else
+#  define __END_DECLS /* nothing */
+# endif
+#endif
+
+#ifndef __GNUC_PREREQ
+# if defined __GNUC__ && defined __GNUC_VERSION__
+# define __GNUC_PREREQ(maj, min) \
+        ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
+# else
+#  define __GNUC_PREREQ(maj, min) 0
+# endif
+#endif
+
+#ifndef __THROW
+# if defined __cplusplus && (__GNUC_PREREQ (2,8) || __clang_major__ >= 4)
+#  if __cplusplus >= 201103L
+#   define __THROW      noexcept (true)
+#  else
+#   define __THROW      throw ()
+#  endif
+# else
+#  define __THROW
+# endif
+#endif
+
+#endif /* _GETOPT_CDEFS_H */