Work around autoconf 2.60 requirement due to use of AC_PROG_SED.

We currently require autoconf 2.58 only but AC_PROG_SED used by
AX_PATH_GENERIC internally is only defined by autoconf 2.60. To avoid
requiring a newer autoconf version just for this macro, predefine it as just
"sed" for the old versions.

This fixes the build with autoconf 2.58.

git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk@12216 626c5289-ae23-0410-ae9c-e8d60b6d4f22

configure.in

@@ -3,7 +3,11 @@ dnl The macros which aren't shipped with the autotools are stored in the
 dnl Tools/config directory in .m4 files.
 
 AC_INIT([swig],[2.0.1],[http://www.swig.org])
+
+dnl NB: When this requirement is increased to 2.60 or later, AC_PROG_SED
+dnl     definition below can be removed
 AC_PREREQ(2.58)
+
 AC_CONFIG_SRCDIR([Source/Swig/swig.h])
 AC_CONFIG_AUX_DIR([Tools/config])
 AC_CONFIG_HEADERS([Source/Include/swigconfig.h])
@@ -50,6 +54,11 @@ AC_CHECK_FUNC(popen, AC_DEFINE(HAVE_POPEN, 1, [Define if popen is available]), A
 fi
 
 dnl PCRE
+
+dnl AX_PATH_GENERIC() relies on AC_PROG_SED() but it is defined only in
+dnl autoconf 2.60 so trivially predefine it ourselves for the older versions
+m4_ifdef([AC_PROG_SED],, [AC_DEFUN([AC_PROG_SED], [AC_PATH_PROG([SED], sed)])])
+
 AC_ARG_WITH([pcre],
   [AS_HELP_STRING([--without-pcre],
                   [Disable support for regular expressions using PCRE])],