Should be <et/com_err.h>
Patch to be attached shortly. I had one for configure.in, but it only checked and didn't set the proper values where they were being used, so I'm working on that now.
Created attachment 206980 [details, diff]
A pkgconfig patch to configure.in would have been nicer.
The same bug and patch apply to postgresql-server.
Committed, thanks for the patch