string_util.c: provide fallback strndup() implementation

This patch also adds extern "C" { } wrapper around string_util.h
to allow its usage in C++ code

src/string_util.c

@@ -20,6 +20,8 @@
 #include "config.h"
 #include "string_util.h"
 
+#include <stdlib.h> /* for malloc() */
+#include <string.h> /* for strnlen() */
 #include <glib.h>
 
 #include <assert.h>
@@ -45,3 +47,22 @@ string_array_contains(const char *const* haystack, const char *needle)
 
 	return false;
 }
+
+#if !defined(HAVE_STRNDUP)
+
+char *
+strndup(const char *str, size_t n)
+{
+	assert(str != NULL);
+
+	size_t len = strnlen(str, n);
+	char* ret = (char *) malloc(len + 1);
+	if (ret == NULL)
+		return NULL;
+
+	memcpy(ret, str, len);
+	ret[len] = '\0';
+	return ret;
+}
+
+#endif