mdbx: release v0.11.2

Acknowledgements:
-----------------

 - [장세연 (Чан Се Ен)](https://github.com/sasgas) for contributing to C++ API.
 - [Alain Picard](https://github.com/castortech) for [Java bindings](https://github.com/castortech/mdbxjni).
 - [Alex Sharov](https://github.com/AskAlexSharov) for reporting and testing.
 - [Kris Zyp](https://github.com/kriszyp) for reporting and testing.
 - [Artem Vorotnikov](https://github.com/vorot93) for support [Rust wrapper](https://github.com/vorot93/libmdbx-rs).

Fixes:
------

 - [Fixed compilation](erthink/libmdbx#239) with `devtoolset-9` on CentOS/RHEL 7.
 - [Fixed unexpected `MDBX_PROBLEM` error](erthink/libmdbx#242) because of update an obsolete meta-page.
 - [Fixed returning `MDBX_NOTFOUND` error](erthink/libmdbx#243) in case an inexact value found for `MDBX_GET_BOTH` operation.
 - [Fixed compilation](erthink/libmdbx#245) without kernel/libc-devel headers.

Minors:
-------

 - Fixed `constexpr`-related macros for legacy compilers.
 - Allowed to define 'CMAKE_CXX_STANDARD` using an environment variable.
 - Simplified collection statistics of page operation .
 - Added `MDBX_FORCE_BUILD_AS_MAIN_PROJECT` cmake option.
 - Remove unneeded `#undef P_DIRTY`.

Signed-off-by: Leonid Yuriev <leo@yuriev.ru>

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