Use the Autoconf macro for finding sed

The AC_PROG_SED macro was added in Autoconf 2.59b and the minimum
supported Autoconf version is now 2.63 so we can safely assume it is
available.

configure.ac

@@ -194,14 +194,12 @@ AC_PROG_CC
 AC_PROG_CPP
 AC_PROG_MAKE_SET
 AC_PROG_RANLIB
+AC_PROG_SED
 
 AC_ARG_VAR([PERL], [path to Perl binary])
 AC_CHECK_PROGS([PERL], [perl])
 AM_CONDITIONAL(USE_PERL, [test "x$ac_cv_prog_PERL" != "x"])
 
-dnl autoconf 2.59 appears not to support AC_PROG_SED
-AC_CHECK_PROG([SED],[sed],[sed],[/bin/false])
-
 dnl check for asciidoc and a2x
 AC_PATH_PROG([ASCIIDOC], [asciidoc], none)
 AC_PATH_PROGS([A2X], [a2x a2x.py], none)