error installing stand/i386/loader_4th in HEAD

I got a very strange error while trying to make installworld on HEAD:

stand/i386/loader_4th (install)
/tmp/install.rIhHqnPC/sh: cc not found
*** Error code 127

This from a tree which I updated at 08:37 UTC.  According to uname -a
the tree is at commit main-n249276-fc29a8b9d95.

I booted the new kernel from this tree.

I've never seen this error before.  Why in the world would install need
cc?

Note that I copied the contents of /usr/src/etc/mtree to /etc/mtree before
starting the installation.

-- 
Gary Jennejohn