Starting with Wireshark 3.0.0, autotools is no longer supported, only cmake. I am surprised that there's no mention of this in the Wireshark 3.0.0 Release Notes, but I guess the release notes is geared more for users than developers.
That said, the Wireshark developer's guide does reflect the new build process:
3.5.1. Building on Unix
The recommended (and fastest) way to build Wireshark is with CMake and Ninja:
# Starting from your Wireshark source directory, create a build directory
# alongside it.
$ cd ..
$ mkdir wireshark-ninja
$ cd wireshark-ninja
# Assumes your source directory is named "wireshark".
$ cmake -G Ninja ../wireshark
$ ninja (or cmake --build .)
If you need to build with a non-standard configuration, you can run
$ cmake -LH ../wireshark
to see what options you have.
I hope that helps.
Looking at the diff between the unpacked 2.6.7 and 3.0.0 directories, it's much more than just two missing scripts. Perhaps the problem is that I am trying to use 2.6 build instructions for 3.0. I think I'll stick with 2.6 for now.
Yes, that's the problem, as per cmaynard's comment.