Eventually, (a long time later) I happened upon the Tool Chain Editor page. Not a page you normally ever need to visit. Sure enough, the Current toolchain was set to NetBurner Device and the Current builder was set to Release ELF builder (MINGW). Even worse, the option for NetBurner Device Library wasn't in the list. Luckily unchecking the Display compatible toolchains only brought that option (and others) back. I was able to reset it to a Device Library and after a rebuild my library built once again without problems. Whew.
I'm actually pretty confident about what caused the problem. On a separate application I had gone into the Properties, Paths and Symbols and on the References tab checked a couple of libraries that the application depended upon. I was hoping this would make it rebuild the application when I hit the Build icon when the dependent libraries had been changed. It didn't work( although it's supposed to) so I unchecked the libraries, naively assuming all would return to how it was. Didn't happen and it cost me an hour. UPDATE : Note there are two ways to get to this same References panel.You can get there as shown in the image below but you can also go there directly with the Project References tab. Both methods cause the heartache described above.