Can't find unicode/ustring.h in test

I get this error during compilation:

make[3]: Entering directory '/home/hacksaw/src/dovecot-2.4.4/src/lib-language' CC test-lang-icu.o In file included from test-lang-icu.c:8: lang-icu.h:4:10: fatal error: unicode/ustring.h: No such file or directory 4 | #include <unicode/ustring.h> | ^~~~~~~~~~~~~~~~~~~ compilation terminated.

I have unicode 66, at least as far as Ubuntu is concerned.