PATCH: add comments explaining DEJAGNU in configure.ac

This may help someone in the future who ends up wondering why configure 
substitutes DEJAGNU.

---
 ChangeLog    |    4 ++++
 configure.ac |    2 ++
 2 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 6dd42cc..20957c1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2018-10-30  Jacob Bachmeyer  <[email protected]>
+
+       * configure.ac (DEJAGNU): Add comments.
+
 2018-10-30  Ben Elliston  <[email protected]>
 
        * Makefile.am (DISTCLEANFILES): Add testrun.xml.
diff --git a/configure.ac b/configure.ac
index 10a0d84..79fefbd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -48,10 +48,12 @@ else
   AC_MSG_ERROR([Tcl 8.5 or greater is required])
 fi
 
+dnl Provide an empty global site file (/dev/null) as default.
 if test x"$DEJAGNU" = x
 then
   DEJAGNU="/dev/null"
 fi
 AC_SUBST(DEJAGNU)
+dnl Makefile.am arranges for DEJAGNU to be exported in the environment.
 
 AC_OUTPUT([Makefile])
-- 


-- Jacob