Fix build using -std=c99
In each place where POSIX/GNU functions are used, the file must declare
that it wants POSIX functionality before including any system headers.
diff --git a/programs/ssl/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 04b847a..74d5d72 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -19,6 +19,11 @@
* This file is part of mbed TLS (https://tls.mbed.org)
*/
+/* Enable definition of hostname() even when compiling with -std=c99. Must be
+ * set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 200112L
+
#if !defined(MBEDTLS_CONFIG_FILE)
#include "mbedtls/config.h"
#else