Fix windows.h inclusion for crosses to MinGW

[Dave Korn]

On 23/03/2011 17:09, Joseph S. Myers wrote:
> The ld configure test for plugins on Windows includes Windows.h, as does 
> plugin.c.  But the header included in MinGW's w32api distribution is 
> actually windows.h not Windows.h, so the configure test fails when the 
> host is MinGW but the build system has a case-sensitive filesystem.  This 
> patch fixes the test and include to use windows.h.  OK to commit?

  I don't have approval rights, but this is clearly correct and borders on
"obvious fix".

    cheers,
      DaveK