fix compile error.

Hello!

My compile problem is solved, make sure you can compile OpenFonts directory successfully. If not, please consider pull my modified branch.

my XeTeX version info:
```
XeTeX 3.1415926-2.2-0.9995.2 (TeX Live 2009/Debian)
kpathsea version 5.0.0
Copyright 2009 SIL International and Jonathan Kew.
There is NO warranty.  Redistribution of this software is
covered by the terms of both the XeTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the XeTeX source.
Primary author of XeTeX: Jonathan Kew.
Compiled with ICU version 3.8.1 [with modifications for XeTeX]
Compiled with zlib version 1.2.3.4; using 1.2.3.4
Compiled with FreeType2 version 2.4.8; using 2.4.8
Compiled with fontconfig version 2.8.0; using 2.8.0
Compiled with libpng version 1.2.46; using 1.2.46
Compiled with poppler version 0.18.2
```

The commit abstract:
=================
Use argument 'ExternalLocation' instead of 'Path' in fontspec commands to fix xelatex compile errors in my case.