CXX-3296 fix build if GCC/GNU/Clang/MSVC macro defined

[kevinAlbs]

Summary

Fix build if GCC/GNU/Clang/MSVC macros are already defined.

Tested with this patch build.

Background

https://www.mongodb.com/community/forums/t/error-compiling-some-c-classes-with-driver-v4-0/321847 reports a build error due to the macro GCC being defined. I can reproduce the error by adding #define GCC 1 before including C++ driver headers:

/Users/kevin.albertson/code/cxx-bootstrap/install/mongo-cxx-driver-r4.1.0/include/bsoncxx/v_noabi/bsoncxx/document/value.hpp:185:9: error: use of undeclared identifier 'BSONCXX_PRIVATE_WARNINGS_DISABLE_IMPL_FOR_1'
        BSONCXX_PRIVATE_WARNINGS_DISABLE(GCC("-Wmaybe-uninitialized"));
        ^

This PR changes the BSONCXX_PRIVATE_WARNINGS_DISABLE* macros to avoid assuming GCC/Clang/MSVC are not already defined:

// Before:
BSONCXX_PRIVATE_WARNINGS_DISABLE(GCC("-Wmaybe-uninitialized"));
// After:
BSONCXX_PRIVATE_WARNINGS_DISABLE_FOR_GCC("-Wmaybe-uninitialized");

This PR assumes the BSONCXX_PRIVATE_* macros are OK to rename in a non-major API release. The macros are in a public header, but documented as private (refer: API Versioning).

[eramongodb]

Was adding macro selection suffixes (Clang, GCC, GNU, and MSVC) to the list of guarded macros considered?

// #pragma push_macro("GCC")
// #pragma undef GCC
#include <bsoncxx/v1/detail/prelude.hpp>

... // Public header code.

// #pragma pop_macro("GCC")
#include <bsoncxx/v1/detail/postlude.hpp>

I do not think we reasonably expect any platforms or third party libraries to define any of these macros (as they are usually prefixed with at least one underscore _) in such a way that guarding them within our public headers would change library behavior. I would consider support for users building the C++ Driver libraries with such macros defined to be out-of-scope (they should be user-defining such macros in their code, not ours), but that doesn't seem to be the case for the referenced user report (diagnostic points to public header code which should be guarded).

[kevinAlbs]

Was adding macro selection suffixes (Clang, GCC, GNU, and MSVC) to the list of guarded macros considered?

Thank you for the suggestion. That seems to fit the purpose of the macro guards and is a smaller change. Updated.

[eramongodb]

Minor suggestion; otherwise, LGTM.

[eramongodb]

Suggest keeping this list alphabetically sorted (+ consistent with the order of pragmas in macro guard headers).

[vector-of-bool]

LGTM