Remove AC_HEADER_STDC replacement code

Remove replacement code added in by autoupdate.
Go with recommendation to unconditionally include C headers
(which we have been doing all along).

Minimum autoconf version can be restored back to what it was.

We do need AC_PROG_EGREP - used by AC_EGREP_CPP

configure.ac

@@ -3,7 +3,7 @@ dnl The macros which aren't shipped with the autotools are stored in the
 dnl Tools/config directory in .m4 files.
 
 AC_INIT([swig],[4.1.0],[http://www.swig.org])
-AC_PREREQ([2.71])
+AC_PREREQ([2.60])
 
 AC_CONFIG_SRCDIR([Source/Swig/swig.h])
 AC_CONFIG_AUX_DIR([Tools/config])
@@ -34,17 +34,9 @@ AC_MSG_RESULT([$CXXFLAGS])
 AC_DEFINE_UNQUOTED(SWIG_CXX, ["$CXX"], [Compiler that built SWIG])
 AC_DEFINE_UNQUOTED(SWIG_PLATFORM, ["$host"], [Platform that SWIG is built for])
 
-dnl Checks for header files.
-m4_warn([obsolete],
-[The preprocessor macro `STDC_HEADERS' is obsolete.
-  Except in unusual embedded environments, you can safely include all
-  ISO C90 headers unconditionally.])dnl
-# Autoupdate added the next two lines to ensure that your configure
-# script's behavior did not change.  They are probably safe to remove.
-AC_CHECK_INCLUDES_DEFAULT
+# For AC_EGREP_CPP
 AC_PROG_EGREP
 
-
 dnl Look for popen
 AC_ARG_WITH(popen, AS_HELP_STRING([--without-popen], [Disable popen]), with_popen="$withval")
 if test x"${with_popen}" = xno ; then