--- a/configure.ac
+++ b/configure.ac
@@ -508,6 +508,8 @@ DEFAULT_VALGRINDFLAGS='-q --error-exitco
 
 gl_VALGRIND_TESTS_DEFAULT_NO
 
+gl_LIBUNISTRING_OPTIONAL
+
 dnl Note that g*l_INIT are run after we check for library capabilities,
 dnl to prevent issues from caching lib dependencies. See discussion
 dnl in https://bugs.gentoo.org/show_bug.cgi?id=494940 and
