configure.ac: update Flex warning language

Make the Flex "you do not need Flex if..." warning to be consistent
with the language about Pandoc.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

configure.ac

@@ -976,7 +976,7 @@ if test -z "$LEX" || \
         AC_MSG_WARN([*** Could not find Flex on your system.])
         AC_MSG_WARN([*** Flex is required for developer builds of Open MPI.])
         AC_MSG_WARN([*** Other versions of Lex are not supported.])
-        AC_MSG_WARN([*** YOU DO NOT NEED FLEX WHEN BUILDING DISTRIBUTION TARBALLS!])
+        AC_MSG_WARN([*** NOTE: If you are building a tarball downloaded from www.open-mpi.org, you do not need Flex])
         AC_MSG_ERROR([Cannot continue])
     fi
 fi