Refactor the way HAVE_XXX macros are set
https://bugs.webkit.org/show_bug.cgi?id=108132

Patch by Laszlo Gombos <l.gombos@samsung.com> on 2013-02-10
Reviewed by Benjamin Poulain.

OS(WINDOWS) and OS(UNIX) are so broadly defined that for each
builds exactly one of them is enabled. Use this assumption to
cleanup Platform.h.

* wtf/Platform.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142399 268f45cc-cd09-0410-ab3c-d52691b4dbfc