on a downgrade libtool relinks libopengine.so.0 against the libopensync.so.1 in /usr/lib/ which is about to be removed.
Seems upstream didn't understand autotools back when it was using them.
opensync/Makefile.am and osengine/Makefile.am:
in both @PACKAGE_LIBS@ should be appended to _LIBADD, not _LDFLAGS
(that '-R $(libdir)' seems redundant too).
Thanks for the hint. It looks like I exposed this when I started running eautoreconf.