Raise message severity up to warning, when strerror_r() exists but

configure is unable of finding out if it has glibc-style or POSIX-style,
trying to ensure that this condition does not go unnoticed.

m4/curl-functions.m4

@@ -22,7 +22,7 @@
 #***************************************************************************
 
 # File version for 'aclocal' use. Keep it a single number.
-# serial 25
+# serial 26
 
 
 dnl CURL_INCLUDES_ARPA_INET
@@ -2692,7 +2692,7 @@ AC_DEFUN([CURL_CHECK_FUNC_STRERROR_R], [
   #
   if test "$tst_compi_strerror_r" = "yes" &&
      test "$tst_allow_strerror_r" = "unknown"; then
-    AC_MSG_NOTICE([cannot determine strerror_r() style: edit lib/config.h manually.])
+    AC_MSG_WARN([cannot determine strerror_r() style: edit lib/config.h manually.])
   fi
   #
 ])