The CMake-based build relies on __has_include, which is standardized in
C++17. Unfortunately, __has_include is available without requiring
--std=c++17 on all the compilers on CI, so this problem was not caught.

Fixes https://github.com/google/leveldb/issues/572

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192208842

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190262375

C++11 requires <atomic>. This lets us remove the header detection
(LEVELDB_ATOMIC_PRESENT) and simplify port/atomic_pointer.h.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189919098

This CL switches the public headers to C++11 default and deleted constructors, and adds override to the relevant leveldb::EnvWrapper methods. This should be a good test for C++11 compiler support.

Once this CL settles, the rest of the codebase can be safely modernized to C++11.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189873212

The header is missing from the file lists, resulting in incomplete installs.

This was originally contributed in https://github.com/google/leveldb/pull/568.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189822907

Fixes https://github.com/google/leveldb/issues/466

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189425354