Well, I finally figured it out. "Way back when", I had manually modified RTCLib.cpp/.h to use SBWire vs Wire libraries. I thought I had done it very cleverly with an added #define in the .h file and #ifdef's in the .cpp file to switch between SBWire.h & Wire.h, but apparently my later self was too clever for my earlier self, or maybe the other way around
. Anyway, the #include SBWire line was still active, and this was causing the compile errors.
BTW, the way I finally conquered the way-too-voluminous 'verbose' compile output was to copy/paste the output into Notepad++, squeeze the font size down to TEEEENSSSyyy, and then spread the result across both of my 24" 2560 x 1440 monitors - and it STILL wasn't enough to prevent line wrap (and turning off line-wrap wasn't the answer either)!
However, it WAS enough to show me that a large part of each line was identical from line to line, so I could use Notepad++'s ALT-Select feature to remove the identical 'boilerplate' text from all lines, leaving only the portion actually relevant to the problem.
From this I was able to tell that each line added just one more library file to the mix, so I was able to follow the progress and carefully work my way through the library files in the same progression as the compiler. Then I was able to find and fix my #include error in RTCLib and all was well. Whew!
Since I have had to to this several times now after descending into "#include hell" on different projects, I begin to wonder if there are some tools out there that can munch through an Arduino-style verbose output and selectively reduce the clutter to the point where a mere mortal such as myself could actually understand it. Anyone know of such an animal? If not, I'm seriously considering building such a tool myself. Thoughts?
Frank