bootstrap: Check support for unordered_map from compiler mode

Some versions of clang 5 (with libc++) have a problem with unordered_map under -std=gnu++1z:

/usr/include/c++/__hash_table:1134:43: error: conflicting types for '__hash_table<_Tp, _Hash, _Equal, _Alloc>'

Include unordered_map in our test source so that we reject this combination and fall back to an older C++ standard flag.

Fixes: #17526 (closed)