ld fails on Ubuntu 21.10 and 22.04 (it succeeds on 20.04). The relevant bits from the buildlog:
libtool: link: gcc -shared -fPIC -DPIC src/.libs/purple_libnotify__la-purple-libnotify+-frames.o src/.libs/purple_libnotify__la-purple-libnotify+-utils.o src/.libs/purple_libnotify__la-purple-libnotify+.o /usr/lib/x86_64-linux-gnu/libpurple-events.so -lpurple -lnotify -lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0 -pthread -pthread -g -O2 -flto=auto -flto=auto -fstack-protector-strong -Wl,-Bsymbolic-functions -flto=auto -flto=auto -Wl,-z -Wl,relro -Wl,-z -Wl,now -pthread -Wl,-soname -Wl,purple-libnotify+.so -Wl,-version-script -Wl,.libs/purple-libnotify+.ver -o .libs/purple-libnotify+.so
/usr/bin/ld: src/.libs/purple_libnotify__la-purple-libnotify+-utils.o (symbol from plugin): in function `notify_plus_data':
(.text+0x0): multiple definition of `notify_plus_data'; src/.libs/purple_libnotify__la-purple-libnotify+-frames.o (symbol from plugin):(.text+0x0): first defined here
/usr/bin/ld: src/.libs/purple_libnotify__la-purple-libnotify+.o (symbol from plugin): in function `notify_plus':
(.text+0x0): multiple definition of `notify_plus_data'; src/.libs/purple_libnotify__la-purple-libnotify+-frames.o (symbol from plugin):(.text+0x0): first defined here
collect2: error: ld returned 1 exit status
make[2]: *** [Makefile:573: purple-libnotify+.la] Error 1
ld fails on Ubuntu 21.10 and 22.04 (it succeeds on 20.04). The relevant bits from the buildlog: