build: enhance clang++ C++20 usability test

clang++-17 -std=c++23 -stdlib=libstdc++ (from gcc-14) fails template
deduction somehow. (Works when using v17 + -std=c++20, or v18 +
-std=any).

```
In file included from x.cpp:1:
In file included from /usr/include/c++/14/string:51:
In file included from /usr/include/c++/14/bits/stl_algobase.h:64:
/usr/include/c++/14/bits/stl_pair.h:410:35: error: no matching function for call to 'get'
  410 |           return _S_convertible<decltype(std::get<0>(std::declval<_UPair>())),
/usr/include/c++/14/bits/stl_pair.h:524:22: note: in instantiation of function template specialization 'std::pair<const int, int>::_S_convertible_from_pair_like<const std::pair<const int, int> *&>' requested here
  524 |         constexpr explicit(!_S_convertible_from_pair_like<_UPair>())
/usr/include/c++/14/type_traits:1110:49: note: while substituting deduced template arguments into function template 'pair' [with _UPair = const std::pair<const int, int> *&]
 1110 |       = __bool_constant<__is_constructible(_Tp, _Args...)>;
/usr/include/c++/14/type_traits:1116:16: note: in instantiation of template type alias '__is_constructible_impl' requested here
 1116 |       : public __is_constructible_impl<_Tp, _Args...>
/usr/include/c++/14/bits/hashtable_policy.h:1140:33: note: in instantiation of template class 'std::is_constructible<std::pair<const int, int>, const std::pair<const int, int> *&>' requested here
 1140 |         using _IFcons = std::enable_if<__is_cons<_Pair>::value>;
/usr/include/c++/14/bits/hashtable_policy.h:1143:2: note: in instantiation of template type alias '_IFcons' requested here
 1143 |         using _IFconsp = typename _IFcons<_Pair>::type;
/usr/include/c++/14/bits/hashtable_policy.h:1153:43: note: (skipping 1 context in backtrace; use -ftemplate-backtrace-limit=0 to see all)
 1153 |       template<typename _Pair, typename = _IFconsp<_Pair>>
/usr/include/c++/14/bits/hashtable_policy.h:1155:2: note: in instantiation of default argument for 'insert<const std::pair<const int, int> *&>' required here
 1155 |         insert(const_iterator __hint, _Pair&& __v)
 1156 |         {
 1157 |           __hashtable& __h = this->_M_conjure_hashtable();
 1158 |           return __h._M_emplace(__hint, __unique_keys{},
 1159 |                                 std::forward<_Pair>(__v));
 1160 |         }
/usr/include/c++/14/bits/hashtable.h:1274:15: note: while substituting deduced template arguments into function template 'insert' [with _Pair = const std::pair<const int, int> *&, $1 = (no value)]
 1274 |       { this->insert(__f, __l); }
/usr/include/c++/14/bits/hashtable.h:585:9: note: in instantiation of function template specialization 'std::_Hashtable<int, std::pair<const int, int>, std::allocator<std::pair<const int, int>>, std::__detail::_Select1st, std::equal_to<int>, std::hash<int>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true>>::_Hashtable<const std::pair<const int, int> *>' requested here
  585 |       : _Hashtable(__l.begin(), __l.end(), __bkt_count_hint,
/usr/include/c++/14/bits/unordered_map.h:239:9: note: in instantiation of member function 'std::_Hashtable<int, std::pair<const int, int>, std::allocator<std::pair<const int, int>>, std::__detail::_Select1st, std::equal_to<int>, std::hash<int>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true>>::_Hashtable' requested here
  239 |       : _M_h(__l, __n, __hf, __eql, __a)
x.cpp:3:30: note: in instantiation of member function 'std::unordered_map<int, int>::unordered_map' requested here
    3 | std::unordered_map<int, int> a{{}};
```

Author: jengelh

configure.ac

@@ -34,10 +34,12 @@ for i in "" "c++26" "c++2c" "c++23" "c++2b" "c++20"; do
 	CXXFLAGS="$saved_CXXFLAGS${i:+ -std=$i}"
 	AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 		#include <span>
+		#include <unordered_map>
 		#include <vector>
 		int main() {
 			std::vector<int> v{1,9};
 			std::span s = v;
+			std::unordered_map<int, int> um{{}};
 		}
 	])], [
 		AC_MSG_RESULT([yes])