Description
Since image version 20210510 both clang-3.9 and clang-9 are broken, specifically compilation errors in the C++ stdlib and binaries that segfault.
With no changes to code or CI 20210504.1 does not exhibit this issue, hence this must be a bug in this repo.

Area for Triage:
C/C++

Question, Bug, or Feature?:
Bug

Virtual environments affected

• Ubuntu 18.04

Image version
Version: 20210510.0

Expected behavior
Clang 3.9 and 9 compile C++ code without erroring and/or the resulting binaries segfaulting.
This is the case on 20210504.1 (https://github.com/minetest/minetest/runs/2535472812).

Actual behavior

• Clang 3.9 throws the following warning on every C++ source file: (https://github.com/minetest/minetest/pull/11263/checks?check_run_id=2575483229)

[...]
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/cstdint:38:
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/x86_64-linux-gnu/c++/11/bits/c++config.h:558:2: warning: "__STRICT_ANSI__ seems to have been undefined; this is not supported" [-W#warnings]
#warning "__STRICT_ANSI__ seems to have been undefined; this is not supported"
 ^
1 warning generated.

...also leading to errors in some cases:

In file included from /home/runner/work/minetest/minetest/src/serverenvironment.cpp:20:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/algorithm:61:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/stl_algobase.h:63:
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/ext/numeric_traits.h:64:7: error: static_assert failed "invalid specialization"
      static_assert(__is_integer_nonstrict<_Value>::__value,
      ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/uniform_int_dist.h:251:19: note: in instantiation of template class '__gnu_cxx::__numeric_traits_integer' requested here
          static_assert(!_Wp_traits::__is_signed, "W must be unsigned");
                         ^admittedly 
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/uniform_int_dist.h:307:11: note: in instantiation of function template specialization 'std::uniform_int_distribution::_S_nd, unsigned long>' requested here
                __ret = _S_nd(__urng, __u64erange);
                        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/stl_algo.h:3791:32: note: in instantiation of function template specialization 'std::uniform_int_distribution::operator() >' requested here
        std::iter_swap(__i, __first + __d(__g, __p_type(0, __i - __first)));
                                      ^
/home/runner/work/minetest/minetest/src/serverenvironment.cpp:1438:8: note: in instantiation of function template specialization 'std::shuffle<__gnu_cxx::__normal_iterator *, std::vector, std::allocator > > >, std::mersenne_twister_engine &>' requested here
                std::shuffle(output.begin(), output.end(), m_rgen);
                     ^

• Clang 9 built binaries segfault for no good reason: https://github.com/minetest/minetest/pull/11263/checks?check_run_id=2575482918
  (admittedly I have not narrowed this down but neither the code nor our CI changed so I'm sure this is related)

Repro steps
Here's the relevant parts of our CI:
https://github.com/minetest/minetest/blob/master/.github/workflows/build.yml#L75-L120
https://github.com/minetest/minetest/blob/master/util/ci/common.sh
https://github.com/minetest/minetest/blob/master/util/ci/build.sh

(see third post)