Define explicit_bzero using memset_s on macOS

chat.h

@@ -295,6 +295,3 @@ int getopt_config(
 	int argc, char *const *argv,
 	const char *optstring, const struct option *longopts, int *longindex
 );
-
-// Defined in libcrypto if missing from libc:
-void explicit_bzero(void *b, size_t len);

configure

@@ -45,6 +45,12 @@ case "$(uname)" in
 		config libcrypto libtls ncursesw
 		defvar OPENSSL_BIN openssl exec_prefix /bin/openssl
 		;;
+	(Darwin)
+		cflags -D__STDC_WANT_LIB_EXT1__=1
+		cflags "-D'explicit_bzero(b,l)=memset_s((b),(l),0,(l))'"
+		config libcrypto libtls ncursesw
+		defvar OPENSSL_BIN openssl exec_prefix /bin/openssl
+		;;
 	(*)
 		config libcrypto libtls ncursesw
 		defvar OPENSSL_BIN openssl exec_prefix /bin/openssl