Add a comment explaining why there is an empty Makefile.am in this

tree.

This commit was SVN r20184.

ompi/mca/common/Makefile.am

@@ -16,3 +16,9 @@
 # $HEADER$
 #
 
+# Note that this file must exist, even though it is empty (there is no
+# "base" directory for the common framework).  autogen.sh and
+# ompi_mca.m4 assume that every framework has a top-level Makefile.am.
+# We *could* adjust the framework glue code to exclude "common" from
+# this requirement, but it's just a lot easier to have an empty
+# Makefile.am here.