Make an attempt to detect missing MTREE files in distrib-dirs.  Not perfect, but this is just a developer seatbelt.
Make an attempt to detect missing MTREE files in distrib-dirs.  Not
perfect, but this is just a developer seatbelt.

PR:		conf/176897
Submitted by:	Garrett Cooper <yaneurabeya@gmail.com>
MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase