contrib/wix/defines.wxi

keyword: handle filectx _customcmp

Suggested by Yuya Nishihara:
https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-October/089461.html

Related to issue5364.

<Include>

  <?if $(var.Platform) = "x64" ?>
    <?define IsX64 = yes ?>
  <?else?>
    <?define IsX64 = no ?>
  <?endif?>

</Include>