options: Add platform-independent getopt wrapper header

Add include/libssh/getopt.h that transparently includes the system
<getopt.h> when available, or declares the bundled fallback interface
otherwise.

Signed-off-by: Mingyuan Li <2560359315@qq.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/getopt.h

@@ -0,0 +1,35 @@
+/*
+ * This file is part of the SSH Library
+ *
+ * Copyright (c) 2026 Mingyuan Li <2560359315@qq.com>
+ *
+ * The SSH Library 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, version 2.1 of the License.
+ *
+ * The SSH Library 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 the SSH Library; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+#ifndef LIBSSH_GETOPT_H
+#define LIBSSH_GETOPT_H
+
+#include "config.h"
+
+#ifdef HAVE_GETOPT_H
+#include <getopt.h>
+#else
+/* Bundled getopt fallback (src/external/getopt.c) */
+extern int opterr, optind, optopt, optreset;
+extern char *optarg;
+int getopt(int nargc, char *const nargv[], const char *ostr);
+#endif
+
+#endif /* LIBSSH_GETOPT_H */

src/CMakeLists.txt

@@ -140,6 +140,13 @@ set(libssh_SRCS
   pki_ed25519_common.c
 )
 
+if (NOT HAVE_GETOPT)
+    set(libssh_SRCS
+        ${libssh_SRCS}
+        external/getopt.c
+    )
+endif (NOT HAVE_GETOPT)
+
 if (DEFAULT_C_NO_DEPRECATION_FLAGS)
     set_source_files_properties(known_hosts.c
                                 PROPERTIES