Fix windows.h inclusion for crosses to MinGW

[Alan Modra]

On Wed, Mar 23, 2011 at 05:09:22PM +0000, Joseph S. Myers wrote:
> 	* configure.in: Check for windows.h, not Windows.h.
> 	* configure: Regenerate.
> 	* plugin.c: Include windows.h, not Windows.h.

OK.

-- 
Alan Modra
Australia Development Lab, IBM