Tweak the UPDATING message a bit about the upgrade path.

Add some automation into Makefile.inc1 to to enforce known good
upgrade from source paths.

1 files changed, 7 insertions, 4 deletions

diff --git a/UPDATING b/UPDATING
index 20a82940a5c8..71a952aff9ab 100644
--- a/UPDATING
+++ b/UPDATING
@@ -41,10 +41,13 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 12.x IS SLOW:
 	11.0-RELEASE). These revisions post-date the 10.2 and 9.3 releases, so
 	you'll need to take the unusual step of upgrading to the tip of the
 	stable branch before moving to 11 or -current via a source upgrade.
-	stable/11 and 11.0-RELEASE post-date the fix so you can move from them
-	to -current. This differs from the historical situation where one could
-	upgrade from anywhere on the last couple of stable branches, so be
-	careful.
+	stable/11 and 11.0-RELEASE have working newer compiler. This differs
+	from the historical situation where one could upgrade from anywhere on
+	the last couple of stable branches, so be careful.
+
+	If you're running a hybrid system on 9.x or 10.x with an updated clang
+	compiler or are using an supported external toolchain, the build system
+	will allow the upgrade. Otherwise it will print a reminder.
 
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