Along with the Automake 1.13.x changes in r27790, rename these third

party configure.in scripts to be configure.ac so that Automake stops
complaining about them.

This commit was SVN r27791.

The following SVN revision numbers were found above:
  r27790 --> open-mpi/ompi@675a2f5c48

opal/mca/event/libevent2019/libevent/config.h.in

@@ -1,4 +1,4 @@
-/* config.h.in.  Generated from configure.in by autoheader.  */
+/* config.h.in.  Generated from configure.ac by autoheader.  */
 
 /* Define if libevent should build without support for a debug mode */
 #undef DISABLE_DEBUG_MODE