nghttp2: Skip "Using system-installed" message if using system curl

We only use the system nghttp2 when `CMAKE_USE_SYSTEM_NGHTTP2` is ON and
`CMAKE_USE_SYSTEM_CURL` is OFF.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5f661a9..e8537ed 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -215,6 +215,10 @@
   mark_as_advanced(CMAKE_USE_SYSTEM_KWIML)
 
   # Mention to the user what system libraries are being used.
+  if(CMAKE_USE_SYSTEM_CURL)
+    # Avoid messaging about curl-only dependencies.
+    list(REMOVE_ITEM UTILITIES NGHTTP2)
+  endif()
   foreach(util IN LISTS UTILITIES ITEMS KWIML)
     if(CMAKE_USE_SYSTEM_${util})
       message(STATUS "Using system-installed ${util}")