don't use libicu backend for libxml2

(it's discouraged from being used, the first path always goes through iconv, icu is only the fallback)

config/lib.json

@@ -450,6 +450,7 @@
     },
     "libmemcached": {
         "source": "libmemcached",
+        "cpp-library": true,
         "static-libs-unix": [
             "libmemcached.a",
             "libmemcachedprotocol.a",
@@ -594,7 +595,6 @@
         ],
         "lib-suggests-unix": [
             "xz",
-            "icu",
             "zlib"
         ],
         "lib-depends-windows": [