How to disable toplevel fixincludes?

[Aaron W. LaFramboise]

With the recent fixincludes reorganization, bootstrap now attempts to
build fixincludes on i686-pc-mingw32.  Like collect2, fixincludes
contains some fairly unrepetant Unixisms, and can not be trivially
changed to build on Windows.  How do I disable it, both at the command
line, and in the sources as the default for the target?

Aaron W. LaFramboise