diff --git a/configure.ac b/configure.ac
index ab0f4c45..ffe52753 100644
--- a/configure.ac
+++ b/configure.ac
@@ -676,6 +676,8 @@ AM_CONDITIONAL([EMSCRIPTEN], [test "x$EMSCRIPTEN" != "x"])
 
 AM_CONDITIONAL([NATIVECLIENT], [test "x$NATIVECLIENT" != "x"])
 
+AC_DEFINE([CONFIGURED], [1], [the build system was properly configured])
+
 dnl Libtool.
 
 LT_INIT([dlopen])
diff --git a/src/libsodium/sodium/core.c b/src/libsodium/sodium/core.c
index 87fd6d14..aa23726e 100644
--- a/src/libsodium/sodium/core.c
+++ b/src/libsodium/sodium/core.c
@@ -19,7 +19,7 @@
 #include "private/mutex.h"
 
 #if !defined(_MSC_VER) && 1
-# warning This is unstable, untested, development code.
+# warning *** This is unstable, untested, development code.
 # warning It might not compile. It might not work as expected.
 # warning It might be totally insecure.
 # warning Do not use this in production.
@@ -27,6 +27,12 @@
 # warning Alternatively, use the "stable" branch in the git repository.
 #endif
 
+#if !defined(_MSC_VER) && (!defined(CONFIGURED) || CONFIGURED != 1)
+# warning *** The library is being compiled using an undocumented method.
+# warning This is not supported. It has not been tested, it might not
+# warning work as expected, and performance is likely to be suboptimal.
+#endif
+
 static volatile int initialized;
 
 int