ssl_mail_client: Define _XOPEN_SOURCE=600 for gethostname
Fixes building this program on NetBSD 9.0.
* be set before config.h, which pulls in glibc's features.h indirectly.
* Harmless on other platforms. */
#define _POSIX_C_SOURCE 200112L
+#define _XOPEN_SOURCE 600
#if !defined(MBEDTLS_CONFIG_FILE)
#include "mbedtls/config.h"
