compat.h: introduce compatibility header

Introduce a compat.h housing compatibility definitions
and macros along the lines of commit 7b49b8d having
introduced a compatibility mechanism for Lua.

First use case (and motivation) is the support for
musl (https://www.musl-libc.org/) which doesn't
bother to provide
    char *strndupa(const char *s, size_t n)

Reviewed-by: Stefano Babic <sbabic@denx.de>
Reported-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Christian Storm <christian.storm@siemens.com>

include/compat.h

@@ -0,0 +1,24 @@
+/*
+ * Author: Christian Storm
+ * Copyright (C) 2018, Siemens AG
+ *
+ * SPDX-License-Identifier:     GPL-2.0-or-later
+ */
+
+#pragma once
+
+#ifndef strndupa
+/*
+ * Define char *strndupa(const char *s, size_t n)
+ * for, e.g., musl (https://www.musl-libc.org/)
+ * which does not bother to implement this function.
+ */
+#define strndupa(s, n)                          \
+	(__extension__({                            \
+		const char *__in = (s);                 \
+		size_t __len = strnlen(__in, (n)) + 1;  \
+		char *__out = (char *)alloca(__len);    \
+		__out[__len - 1] = '\0';                \
+		(char *)memcpy(__out, __in, __len - 1); \
+	}))
+#endif

include/util.h

@@ -12,6 +12,7 @@
 #include <string.h>
 #include "swupdate.h"
 #include "swupdate_status.h"
+#include "compat.h"
 
 #define NOTIFY_BUF_SIZE 	2048
 #define ENOMEM_ASPRINTF		-1

ipc/network_ipc.c

@@ -27,6 +27,7 @@
 #include <pthread.h>
 
 #include "network_ipc.h"
+#include "compat.h"
 
 #ifdef CONFIG_SOCKET_CTRL_PATH
 static char* SOCKET_CTRL_PATH = (char*)CONFIG_SOCKET_CTRL_PATH;