build: fix Autoconf macros to check for C++ standard flags

[Akim Demaille]

commit 3cfe87572ecba139c4f01f0406c4f1893045a6cd
Author: Akim Demaille <address@hidden>
Date:   Sun Sep 23 08:28:59 2018 +0200

    build: fix Autoconf macros to check for C++ standard flags
    
    * m4/bison-cxx-std.m4: Since now we link the program, we need a
    program: main was missing and linking was failing.

diff --git a/m4/bison-cxx-std.m4 b/m4/bison-cxx-std.m4
index 8c043e80..43e2a1d2 100644
--- a/m4/bison-cxx-std.m4
+++ b/m4/bison-cxx-std.m4
@@ -84,25 +84,25 @@ m4_define([_BISON_CXX_COMPILE_STDCXX_2A_snippet],
 
 
 m4_define([_BISON_CXX_COMPILE_STDCXX_11_testbody],
-[AC_LANG_SOURCE([
+[AC_LANG_PROGRAM([
 _BISON_CXX_COMPILE_STDCXX_11_snippet
 ])])
 
 m4_define([_BISON_CXX_COMPILE_STDCXX_14_testbody],
-[AC_LANG_SOURCE([
+[AC_LANG_PROGRAM([
 _BISON_CXX_COMPILE_STDCXX_11_snippet
 _BISON_CXX_COMPILE_STDCXX_14_snippet
 ])])
 
 m4_define([_BISON_CXX_COMPILE_STDCXX_17_testbody],
-[AC_LANG_SOURCE([
+[AC_LANG_PROGRAM([
 _BISON_CXX_COMPILE_STDCXX_11_snippet
 _BISON_CXX_COMPILE_STDCXX_14_snippet
 _BISON_CXX_COMPILE_STDCXX_17_snippet
 ])])
 
 m4_define([_BISON_CXX_COMPILE_STDCXX_2A_testbody],
-[AC_LANG_SOURCE([
+[AC_LANG_PROGRAM([
 _BISON_CXX_COMPILE_STDCXX_11_snippet
 _BISON_CXX_COMPILE_STDCXX_14_snippet
 _BISON_CXX_COMPILE_STDCXX_17_snippet