having trouble building r-devel

Aha-- thanks!

Or I've
gotten my wires crossed...

The latter.  Try commenting out

# path (possibly full path) to same version of R on the host system
# R_EXE=R
^^

in MkRules.  It's uncommented there from experiments with 
cross-compiling.
Works for me.

Yes, it's just worked for me too. Is it worth mentioning this in the
INSTALL file after the sentence that says "IMPORTANT: edit MkRules to
set the appropriate paths..."? [It may be that all my builds hitherto
have actually invoked an earlier version of R than the one being built,
but that this never mattered before.]

So it looks like either (i) the documentation should be 

changed to tell

the user to reset the path manually (and NB the dot isn't useful for
Windows 2000), 

It was when I ran Windows 2000.  What's wrong for you?

Just that, at the moment of running "make", the dot refers to
src\gnuwin32, which doesn't pick up the R.exe file. With the
commenting-out above, "make" now works fine for me now whether or not
PATH starts with a dot. [I thought MS-DOS implicitly put a dot in, i.e.
doesn't it always look for EXEs and BATs in the current directory
first?] I think I need to start the path with <<current-R-version>>\bin
rather than dot.