[spurious parallel build bug] make -j fails with: "mv: mv: cannot stat `perl.mak': No such file or directory"

I got this when i built v1.6.3-rc3 for the first time:

$ make -j
[...]
    CC builtin-annotate.o
    CC builtin-apply.o
    CC builtin-archive.o
    CC builtin-bisect--helper.o
mv: mv: cannot stat `perl.mak': No such file or directory
mv: cannot stat `perl.mak': No such file or directory
    CC builtin-blame.o
mv: cannot stat `perl.mak': No such file or directory
mv: cannot stat `perl.mak': No such file or directory
cannot stat `perl.mak': No such file or directory

a plain 'make' worked.

Interestingly, a second attempt to reproduce it after a 'make clean' 
failed to trigger the bug. So it's either timing sensitive or 
there's some other weirdness that caused this.

	Ingo