require c++17 instead of c++11

[bertsky]

Fixes the following build issue on GCC v9:

In file included from leveldb/util/comparator.cc:14:
leveldb/./util/no_destructor.h: In constructor ‘leveldb::NoDestructor<InstanceType>::NoDestructor(ConstructorArgTypes&& ...)’:
leveldb/./util/no_destructor.h:24:24: error: ‘is_standard_layout_v’ is not a member of ‘std’; did you mean ‘is_standard_layout’?
   24 |     static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
      |                        ^~~~~~~~~~~~~~~~~~~~
      |                        is_standard_layout
leveldb/./util/no_destructor.h:24:70: error: expected primary-expression before ‘>’ token
   24 |     static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
      |                                                                      ^~
leveldb/./util/no_destructor.h:24:72: error: expected primary-expression before ‘)’ token
   24 |     static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
      |                                                                        ^
make[2]: *** [CMakeFiles/leveldb.dir/build.make:468: CMakeFiles/leveldb.dir/util/comparator.cc.o] Fehler 1
make[1]: *** [CMakeFiles/Makefile2:183: CMakeFiles/leveldb.dir/all] Fehler 2

This is a C++17 feature.

Note: I won't sign Google CLA.

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[littlemountainman]

Since the google styleguide states "Currently, code should target C++20, i.e., should not use C++23 features" . Seems reasonable.

Had found the same regression as you in #1247

[WHYBBE]

Is there anyone who can merge this request, or create a new PR? It's just a very minor fix.

[acornmonkey]

It is better to make this compatible with C++11 by replacing the std::is_standard_layout_v part, as suggested in https://github.com/google/leveldb/pull/1251/commits/16c2b79d4d27a66b7e9da31117e01343e5b2d991.