This is why the boss at my first sysadmin job insisted that we compile everything from source ourselves. Because package managers only complicate things. Turns out that even though I stripped out the packages and reinstalled the new IDE in my home directory (may the aforementioned BOFH forgive me for ever installing in my home directory) I hadn't actually checked to make sure that ALL the files were deleted. Turns out that there was a copy of the seed version installed in /usr/share/arduino/libraries/. Which wouldn't be a problem, except that even though I installed in my home directory (a thousand pardons sir), the IDE decided to default to the shared library location and was using that library, which has it capitalized. Which is why nothing would compile without the change.
So I deleted them, quit all IDE sessions, reloaded, and tried again. I still required the "MHZ" in my trial program. Rebooting didn't help. Somewhere the IDE was hiding that define with the capital Z, and I couldn't find it.
So I got medieval on it. Removed everything, from everywhere. Downloads, folders, everything. "sudo find /* | grep mcp" found a bunch of stuff hiding in /tmp. Stripped it all. Started over from scratch, redownloading and reinstalling. Deleted all my CAN test sketches (OK, that may have been a bit much, but I was frustrated). Which was a bummer as I had a working three node setup (two nanos and a due) all talking even with all this namespace clashing.
So, after the complete nuke drop and reinstall, I've got the three nodes talking again, without the capital "Z" but this is crazy. I have no idea which of my other projects I've broken by doing this.
I once worked in a shop where the suits decided that Silicon Graphics was the next Sun, and dropped a bunch of IRIX machines on us and told us to use them. We had to port everything. This feels a lot like that, except that was the early 1990's and I was getting paid...
Josh
-- I think the moral of this story is "If you've ever done anything with CAN in the arduino environment before attempting the NMEA2000 stuff, delete it ALL and start from scratch."