Needed with icu-76.1 at least. Not clear why it worked before.

/var/tmp/portage/dev-libs/libxml2-2.11.9/work/libxml2-2.11.9-abi_x86_32.x86/.libs/runtest:
  symbol lookup error: /var/tmp/portage/dev-libs/libxml2-2.11.9/work/libxml2-2.11.9-abi_x86_32.x86/.libs/libxml2.so.2: undefined symbol: UCNV_FROM_U_CALLBACK_STOP
--- a/configure.ac
+++ b/configure.ac
@@ -1108,10 +1108,10 @@ if test "$with_icu" = "no" || test "$with_icu" = "" ; then
 else
     # Try pkg-config first so that static linking works.
     # If this succeeeds, we ignore the WITH_ICU directory.
-    PKG_CHECK_MODULES([ICU], [icu-i18n], [
-        WITH_ICU=1; XML_PC_REQUIRES="${XML_PC_REQUIRES} icu-i18n"
+    PKG_CHECK_MODULES([ICU], [icu-uc icu-i18n], [
+        WITH_ICU=1; XML_PC_REQUIRES="${XML_PC_REQUIRES} icu-uc icu-i18n"
         m4_ifdef([PKG_CHECK_VAR],
-            [PKG_CHECK_VAR([ICU_DEFS], [icu-i18n], [DEFS])])
+            [PKG_CHECK_VAR([ICU_DEFS], [icu-uc icu-i18n], [DEFS])])
         if test "x$ICU_DEFS" != "x"; then
             ICU_CFLAGS="$ICU_CFLAGS $ICU_DEFS"
         fi],[:])