When trying to build the musicbrainz-collate extension on a recent Debian testing, the linking step fails, with various messages about undefined references to main and to library functions, indicating that the linker is trying to produce an executable instead of a shared object.

I was able to get an apparently-working extension by either adding -shared to the end of the command line, or removing -Wl,-z,relro from it. The latter sneaks in because it is in the output of icu-config --ldflags.

As using icu-config is no longer recommended anyway, the best fix seems to be to use pkg-config instead. pkg-config --libs icu-uc icu-i18n output does not contain any special linker flags, just -l. Unfortunately, there were no pc files for ICU yet in the ancient Ubuntu version that is still the official minimum version.