build speedup patch

[DJ Delorie]

Note to Mumit: includes patch to w32api/include/windows.h

I've just checked in a change to winsup.h (and corresponding tweaks
elsewhere) that cuts a third of the time off the cygwin1.dll build.
Basically, we were including every single windows header in every
file, although most weren't needed.  I added some defines to winsup.h
to exclude many of the windows headers, and tweaked the few files that
actually needed more than the usual headers (7 of 63 required
winuser.h, 2 required winnls.h, 1 required winsock.h).

My build times (wall clock time) for new-cygwin1.dll went from six
minutes to four.  (ps: disabling "-g -O2" dropped it further to three
minutes).