Add back U_WCHAR_IS_UTF32 macro to pmac.h...

Add back U_WCHAR_IS_UTF32 macro to pmac.h

That was accidentally dropped when upgrading to ICU 4.6.
That leads to a failure in ICUStringConversionsTest.ConvertCodepageUTF8 
on Mac. 

__STDC_ISO_10646__  is not defined on Mac. Neither is _UCS_4. So, u_strFromWCS
ends up calling wcstombs on Mac instead of u_strFromUTF32. I'm gonna change
WideToCodepage in base/i18n/icu_string*.cc to use u_strFromUTF32 directly.
However, there might be other places where the absence of U_WCHAR_IS_UTF32 can
lead to a problem. To be safe, I'm adding back this to pmac.h

BUG=61514
TEST=With this version of ICU 4.6 pulled in, all the tests pass
including 'base_unittests --gtest_filter=ICU*.Conv*'


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=75773

[Mark Mentovai, 2011-02-23 18:54:54]

LGTM