configure: use AC_MSG_NOTICE() instead of AC_MSG_WARN() to inform user about C++ compiler.
Now './configure --silent && make -s' doesn't print any message to stdout or stderr.
diff --git a/configure.ac b/configure.ac
index 628b8f8..369c7e5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -693,7 +693,7 @@
 fi
 if test "$preset_cxx" != "$CXX"
 then
-        AC_MSG_WARN([
+        AC_MSG_NOTICE([
 
   By default, distutils will build C++ extension modules with "$CXX".
   If this is not intended, then set CXX on the configure command line.