cosmetics: prefer the term "Windows" over "Win32"

cosmetics: prefer the term "Windows" over "Win32"

Microsoft has renamed the Win32 API to "Windows API":
  <http://msdn.microsoft.com/en-us/library/aa383723.aspx>

Also, after some discussion on bug-gnulib, when talking about hosts and
platforms we believe it's better to talk about "Windows" or "native
Windows" instead:
  <https://lists.gnu.org/archive/html/bug-gnulib/2012-01/msg00009.html>
  <https://lists.gnu.org/archive/html/bug-gnulib/2012-01/msg00027.html>

* doc/automake.texi: Mention "Windows", not "Win32".
* lib/Automake/XFile.pm: Likewise.
* lib/ar-lib: Likewise.
* lib/compile: Likewise.
* tests/compile2.test: Mention "Windows", not "w32".