Follow-up to r38851: Fix build with autoconf 2.59.
Follow-up to r38851: Fix build with autoconf 2.59.

* configure.ac: AC_PROG_SED isn't defined in autoconf-2.59, so fall
   back to a reasonable default for $SED in case no sed was found.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion