cleaned up windows header usage (added one central file to use windows / winapi specific stuff, to be able to set the appropriate flags before including it correctly)
Note: this may screw up mingw or cygwin building, ill fix it later...
git-svn-id: https://svn.code.sf.net/p/rathena/svn/trunk@16219 54d463be-8e91-2dee-dedb-b68131a5f0ec