[gnote] Update C++ compiler check macros

[Aurimas ÄŒernius <aurimasc src gnome org>]

commit e8c1f5b03bb656fcd6536c52b3c4ac4bf0089d37
Author: Aurimas ÄŒernius <aurisc4 gmail com>
Date:   Sat May 4 17:32:06 2019 +0300

    Update C++ compiler check macros

 m4/ax_cxx_compile_stdcxx.m4 | 42 ++++--------------------------------------
 1 file changed, 4 insertions(+), 38 deletions(-)
---
diff --git a/m4/ax_cxx_compile_stdcxx.m4 b/m4/ax_cxx_compile_stdcxx.m4
index 5032bba8..9e9eaeda 100644
--- a/m4/ax_cxx_compile_stdcxx.m4
+++ b/m4/ax_cxx_compile_stdcxx.m4
@@ -33,19 +33,18 @@
 #   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>
-#   Copyright (c) 2016 Krzesimir Nowak <qdlacz gmail com>
+#   Copyright (c) 2016, 2018 Krzesimir Nowak <qdlacz gmail com>
 #
 #   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 7
+#serial 10
 
 dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
 dnl  (serial version number 13).
 
-AX_REQUIRE_DEFINED([AC_MSG_WARN])
 AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
   m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
         [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
@@ -61,14 +60,6 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
         [m4_fatal([invalid third argument `$3' to AX_CXX_COMPILE_STDCXX])])
   AC_LANG_PUSH([C++])dnl
   ac_success=no
-  AC_CACHE_CHECK(whether $CXX supports C++$1 features by default,
-  ax_cv_cxx_compile_cxx$1,
-  [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
-    [ax_cv_cxx_compile_cxx$1=yes],
-    [ax_cv_cxx_compile_cxx$1=no])])
-  if test x$ax_cv_cxx_compile_cxx$1 = xyes; then
-    ac_success=yes
-  fi
 
   m4_if([$2], [noext], [], [dnl
   if test x$ac_success = xno; then
@@ -139,7 +130,6 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
               [define if the compiler supports basic C++$1 syntax])
   fi
   AC_SUBST(HAVE_CXX$1)
-  m4_if([$1], [17], [AC_MSG_WARN([C++17 is not yet standardized, so the checks may change in incompatible 
ways anytime])])
 ])
 
 
@@ -587,20 +577,12 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus <= 201402L
+#elif __cplusplus < 201703L
 
 #error "This is not a C++17 compiler"
 
 #else
 
-#if defined(__clang__)
-  #define REALLY_CLANG
-#else
-  #if defined(__GNUC__)
-    #define REALLY_GCC
-  #endif
-#endif
-
 #include <initializer_list>
 #include <utility>
 #include <type_traits>
@@ -608,16 +590,12 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
 namespace cxx17
 {
 
-#if !defined(REALLY_CLANG)
   namespace test_constexpr_lambdas
   {
 
-    // TODO: test it with clang++ from git
-
     constexpr int foo = [](){return 42;}();
 
   }
-#endif // !defined(REALLY_CLANG)
 
   namespace test::nested_namespace::definitions
   {
@@ -852,12 +830,9 @@ namespace cxx17
 
   }
 
-#if !defined(REALLY_CLANG)
   namespace test_template_argument_deduction_for_class_templates
   {
 
-    // TODO: test it with clang++ from git
-
     template <typename T1, typename T2>
     struct pair
     {
@@ -876,7 +851,6 @@ namespace cxx17
     }
 
   }
-#endif // !defined(REALLY_CLANG)
 
   namespace test_non_type_auto_template_parameters
   {
@@ -890,12 +864,9 @@ namespace cxx17
 
   }
 
-#if !defined(REALLY_CLANG)
   namespace test_structured_bindings
   {
 
-    // TODO: test it with clang++ from git
-
     int arr[2] = { 1, 2 };
     std::pair<int, int> pr = { 1, 2 };
 
@@ -927,14 +898,10 @@ namespace cxx17
     const auto [ x3, y3 ] = f3();
 
   }
-#endif // !defined(REALLY_CLANG)
 
-#if !defined(REALLY_CLANG)
   namespace test_exception_spec_type_system
   {
 
-    // TODO: test it with clang++ from git
-
     struct Good {};
     struct Bad {};
 
@@ -952,7 +919,6 @@ namespace cxx17
     static_assert (std::is_same_v<Good, decltype(f(g1, g2))>);
 
   }
-#endif // !defined(REALLY_CLANG)
 
   namespace test_inline_variables
   {
@@ -977,6 +943,6 @@ namespace cxx17
 
 }  // namespace cxx17
 
-#endif  // __cplusplus <= 201402L
+#endif  // __cplusplus < 201703L
 
 ]])