Last night I started pkg_rolling-replace on a system, and it failed to
build graphics/mng in an odd way, not finding a Makefile.  This package
built fine on another system.  I noticed that the regenerated configure
was very short, and that m4 had gone to 1.4.10.  I reverted m4 to
1.4.9nb1 and now mng builds fine.  I had a similar problem with
print/cups and it's now ok as well.

I installed m4 1.4.10 on a second machine and it broke in the same way.

So I think something is wrong with m4 1.4.10, but I haven't a clue what.