configure.ac: Require a C++11 compiler for the optional test suite

.github/workflows/data/expat_config_h_in__expected.txt

@@ -4,6 +4,7 @@ BYTEORDER
 const
 HAVE_ARC4RANDOM
 HAVE_ARC4RANDOM_BUF
+HAVE_CXX11
 HAVE_DLFCN_H
 HAVE_FCNTL_H
 HAVE_GETPAGESIZE

expat/acinclude.m4

@@ -8,5 +8,7 @@ m4_include(conftools/ax-append-flag.m4)
 m4_include(conftools/ax-append-compile-flags.m4)
 m4_include(conftools/ax-append-link-flags.m4)
 m4_include(conftools/expatcfg-compiler-supports-visibility.m4)
+m4_include(conftools/ax-cxx-compile-stdcxx.m4)
+m4_include(conftools/ax-cxx-compile-stdcxx-11.m4)
 
 ### end of file

expat/configure.ac

@@ -180,6 +180,8 @@ AC_ARG_WITH([tests],
   [with_tests=yes])
 AM_CONDITIONAL([WITH_TESTS], [test x${with_tests} = xyes])
 
+AS_IF([test x${with_tests} = xyes],
+      [AX_CXX_COMPILE_STDCXX_11([noext], [mandatory])])
 
 AS_VAR_SET([EXPATCFG_ON_MINGW],[no])
 AS_CASE("${host_os}",

expat/conftools/ax-cxx-compile-stdcxx-11.m4

@@ -0,0 +1,39 @@
+# =============================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html
+# =============================================================================
+#
+# SYNOPSIS
+#
+#   AX_CXX_COMPILE_STDCXX_11([ext|noext], [mandatory|optional])
+#
+# DESCRIPTION
+#
+#   Check for baseline language coverage in the compiler for the C++11
+#   standard; if necessary, add switches to CXX and CXXCPP to enable
+#   support.
+#
+#   This macro is a convenience alias for calling the AX_CXX_COMPILE_STDCXX
+#   macro with the version set to C++11.  The two optional arguments are
+#   forwarded literally as the second and third argument respectively.
+#   Please see the documentation for the AX_CXX_COMPILE_STDCXX macro for
+#   more information.  If you want to use this macro, you also need to
+#   download the ax_cxx_compile_stdcxx.m4 file.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Benjamin Kosnik <bkoz@redhat.com>
+#   Copyright (c) 2012 Zack Weinberg <zackw@panix.com>
+#   Copyright (c) 2013 Roy Stogner <roystgnr@ices.utexas.edu>
+#   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov@google.com>
+#   Copyright (c) 2015 Paul Norman <penorman@mac.com>
+#   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 18
+
+AX_REQUIRE_DEFINED([AX_CXX_COMPILE_STDCXX])
+AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [AX_CXX_COMPILE_STDCXX([11], [$1], [$2])])