Rename DONT_USE_WIN_LINE_METRICS to match the spec

The [OS/2 spec][1] uses the name USE_TYPO_METRICS, so using the same
name here is less confusing.

[1]: http://www.microsoft.com/typography/otspec/os2.htm#fss
Merge pull request #19 from khaledhosny/master

Rename DONT_USE_WIN_LINE_METRICS to match the spec