Allow special characters in the names of transcluded files.

Successfully tested on Windows 7/64 with the following filenames:

- öäüÖÄÜßéèáàôÔÉÈÁÀ.md
- Дмитрий Дмитриевич Шостакович.md
- 艾未未.md

Done with MinGW on Windows 7/64.

The code probably needs revision (memory leaks, coding conventions etc.), C is not my language.