Was my Postgres cluster built with the ICU libraries available for Postgres 10 and later?
Does this mean I have the ICU libraries installed, or not?


The configure option that really brings in the library is --with-icu. Currently, checking for this one seems the more reliable way to tell whether ICU has been included in the build.

ICU_CFLAGS and ICU_LIBS are paths that are required only if the library is installed where the compiler does not search by default.


  Does that happen only if the ICU libraries are in place?


It does, but pg_collation also has a new collprovider field indicating that a collation is from ICU (i) or the libc (c)