[DRAFT] MSVC: Add msvc_crt_link options

[ChrisDenton]

Note
This only applies when libc is used as a dependency of std.

PR is for testing/discussion purposes only. It should not be merged yet.

See rust-lang/rust#39016 for more background

Currently libc hard codes the MSVC CRT that's used by the compiler. Users can select between two CRTs (dynamic and static) by toggling the target-feature=crt-static option. However, this is far from ideal given the number of possible configurations, especially when attempting to co-exist with existing C/C++ builds.

At a minimum, it's also useful to be able to use the debug or release versions of these libraries. The following table shows these possible combinations:

	Release	Debug
Dynamic	msvcrt	msvcrtd
Static	libcmt	libcmtd

Furthermore, users may want to disable linking the crt entirely so that they can fully control what is linked and how.

To support these different configurations, this PR adds a new cfg, msvc_crt_link=option, where option is the link configuration option. This can be set in global rustflags (using --cfg) so that it's applied consistently. The two possible options are:

• debug: Use the debug versions of the CRT libraries
• nocrt: disable all CRT linking. This obviously overrides all other options because nothing is linked

[rustbot]

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[JohnTitor]

Triage: This has been open for more than six months. Is there anything I can help here?

[ChrisDenton]

Hmm... I don't think so, I'd hoped this would be part of a wider discussion on solving the issue but that discussion doesn't look like it's going to happen any time soon. I'll remove this for now and maybe revisit if/when I can gather more interest. It is something that needs broader input so I wouldn't want to move forward without that.