* configure.ac (opsysfile): Tweak message for null case.

(The autoconf manual says this is portable.)

ChangeLog

 2012-07-30  Glenn Morris  <rgm@gnu.org>
 
+	* configure.ac (opsysfile): Tweak message for null case.
+
 	* configure.ac (opsysfile): Set to empty on freebsd, gnu-linux,
 	gnu-kfreebsd; and to usg5-4-common.h on sol2*, unixware.
 

configure.ac

@@ -4298,7 +4298,7 @@ echo "
 Configured for \`${canonical}'.
 
   Where should the build process find the source code?    ${srcdir}
-  What operating system file should Emacs use?            ${opsysfile-none}
+  What operating system file should Emacs use?            ${opsysfile:-none}
   What compiler should emacs be built with?               ${CC} ${CFLAGS}
   Should Emacs use the GNU version of malloc?             ${GNU_MALLOC}${GNU_MALLOC_reason}
   Should Emacs use a relocating allocator for buffers?    ${REL_ALLOC}