New CMake option - CHCPP_EMBEDDED_BUILD

[AndreyAlifanov]

We need ability to build clickhouse-cpp as part of our product. But CMakeLists.txt uses global options, sush as BUILD_SHARED_LIBS, BUILD_TESTS and so on. This breaks our build system.

So I made new flag - CHCPP_EMBEDDED_BUILD. If external code sets this flag (in upper CMakeLists.txt, for example), global options are not changed by clickhouse-cpp.

By default, CHCPP_EMBEDDED_BUILD is not set, so it doesn't affect default behavior.

Usage example. In this example flags touch only clickhouse-cpp, and don't affect our projects.

set (CHCPP_EMBEDDED_BUILD ON)
set (BUILD_SHARED_LIBS OFF)
set (BUILD_BENCHMARK OFF)
set (BUILD_TESTS OFF)
add_subdirectory(clickhouse-cpp)

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