Library:

  I ran reimp on User32.lib from the MS Platform SDK to generate libuser32.a.

Header:

  I used the original WinUser.h from the MS Platform SDK.
  I made some sketchy modifications to the original, however.
     Search for "jake" in WinUser.h to find my changes.

NOTES:
I was getting compiler errors when Mame or PacManFan's program tried to run GetLastError().  The new winuser.h did not have a declaration for GetLastError().  The old Mame MinGW winuser.h did.  I more or less copied the line from the old Mame MinGW winuser.h to the new winuser.h file, and Mame compiled!