I had success by temporarily moving the old version libz files out
of /usr/lib. And it seems to be working well so far. Hooray!
However, once I move the old zlib files back into /usr/lib and
restart mt-daapd, the problems come back.
The final gcc command in src/Makefile (that links mt-daapd) does
include -L/usr/local/lib before it has -lz. Perhaps I could get configure
to specify the actual .so file for zlib as it does for sqlite3 and id3tags.