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

bootstrap

@@ -1056,6 +1056,7 @@ TMPFILE=`cmake_tmp_file`
 echo '
 #include <iostream>
 #include <memory>
+#include <unordered_map>
 
 #if __cplusplus < 201103L
 #error "Compiler is not in a mode aware of C++11."