revert own commit 28662.

strnlen is a GNU extension according to http://unixpapa.com/incnote/string.html

source/blender/blenlib/intern/string.c

@@ -342,3 +342,9 @@ void BLI_timestr(double _time, char *str)
 	str[11]=0;
 }
 
+/* determine the length of a fixed-size string */
+size_t BLI_strnlen(const char *str, size_t maxlen)
+{
+	const char *end = memchr(str, '\0', maxlen);
+	return end ? (size_t) (end - str) : maxlen;
+}