Enable IAccessible for MinGW32, keep IAccessible2 disabled for MinGW

Enable IAccessible for MinGW32, keep IAccessible2 disabled for MinGW

No surprise, the files generated by midl contains code that only
the MS compiler will understand.
However, since the generated files are only needed for IA2, we
can still compile the old IAccessible/MSAA implementation for mingw
(like it was before the introduction of IA2)

I used mingw 4.6.1 to test this.

Change-Id: Iad581f2fee087bd7b796ac5a01548f2fcacbd595
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>