lavu: add av_strtok()

The function strtok_r() is part of the POSIX.1 specification, but is not
available on some platforms. We provide an internal implementation, so we
do not need to rely on a platform implementation.

libavutil/avstring.c

@@ -160,6 +160,35 @@ char *av_get_token(const char **buf, const char *term)
     return ret;
 }
 
+char *av_strtok(char *s, const char *delim, char **saveptr)
+{
+    char *tok;
+
+    if (!s && !(s = *saveptr))
+        return NULL;
+
+    /* skip leading delimiters */
+    s += strspn(s, delim);
+
+    /* s now points to the first non delimiter char, or to the end of the string */
+    if (!*s) {
+        *saveptr = NULL;
+        return NULL;
+    }
+    tok = s++;
+
+    /* skip non delimiters */
+    s += strcspn(s, delim);
+    if (*s) {
+        *s = 0;
+        *saveptr = s+1;
+    } else {
+        *saveptr = NULL;
+    }
+
+    return tok;
+}
+
 #ifdef TEST
 
 #undef printf