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

configure.ac

@@ -135,6 +135,8 @@ AC_SEARCH_LIBS([gethostbyname], [nsl])
 
 AC_CHECK_FUNCS(pipe2 accept4 eventfd)
 
+AC_CHECK_FUNCS(strndup)
+
 AC_SEARCH_LIBS([exp], [m],,
 	[AC_MSG_ERROR([exp() not found])])
 

src/decoder/OpusReader.hxx

@@ -21,6 +21,7 @@
 #define MPD_OPUS_READER_HXX
 
 #include "check.h"
+#include "string_util.h"
 
 #include <stdint.h>
 #include <string.h>

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

src/string_util.h

@@ -23,6 +23,11 @@
 #include "gcc.h"
 
 #include <stdbool.h>
+#include <stdlib.h> /* for size_t */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 /**
  * Remove the "const" attribute from a string pointer.  This is a
@@ -78,4 +83,25 @@ strchug_fast(char *p)
 bool
 string_array_contains(const char *const* haystack, const char *needle);
 
+#if !defined(HAVE_STRNDUP)
+
+/**
+ * Duplicates the string to a newly allocated buffer
+ * copying at most n characters.
+ *
+ * @param str a string to duplicate
+ * @param n maximal number of characters to copy
+ * @return a pointer to the duplicated string,
+ * or NULL if memory allocation failed.
+ */
+gcc_malloc
+char *
+strndup(const char *str, size_t n);
+
+#endif
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif 
+
 #endif