std-gnu11: Make compatible with Autoconf 2.70

[Bruno Haible]

When I bootstrap GNU libunistring with Autoconf 2.70, the generated
'configure' is broken leading to:

checking whether gcc accepts -g... yes
checking for gcc option to enable C11 features... none needed
./configure: line 7012: syntax error near unexpected token 
`ac_cv_prog_cc_stdc=$ac_cv_prog_cc_c89'
./configure: line 7012: `                      
ac_cv_prog_cc_stdc=$ac_cv_prog_cc_c89'

Apparently there is some incompatibility between Gnulib's m4/std-gnu11.m4
and PREFIX/share/autoconf/autoconf/c.m4 from Autoconf 2.70.

This patch fixes it.


2020-12-08  Bruno Haible  <bruno@clisp.org>

        std-gnu11: Make compatible with Autoconf 2.70.
        * m4/std-gnu11.m4: Disable the entire file if Autoconf >= 2.70 is in
        use.

diff --git a/m4/std-gnu11.m4 b/m4/std-gnu11.m4
index db833d8..b5ab8ab 100644
--- a/m4/std-gnu11.m4
+++ b/m4/std-gnu11.m4
@@ -6,6 +6,8 @@
 # This implementation will be obsolete once we can assume Autoconf 2.70
 # or later is installed everywhere a Gnulib program might be developed.
 
+m4_version_prereq([2.70], [], [
+
 
 # Copyright (C) 2001-2020 Free Software Foundation, Inc.
 
@@ -822,3 +824,6 @@ dnl Tru64   N/A (no support)
 dnl with extended modes being tried first.
 [[-std=gnu++11 -std=c++11 -std=gnu++0x -std=c++0x -qlanglvl=extended0x -AA]], 
[$1], [$2])[]dnl
 ])# _AC_PROG_CXX_CXX11
+
+
+])# m4_version_prereq