Build system changes for MinGW support

[Ericson2314]

Motivation

These changes are needed to enable a MinGW build. The most interesting part is support for platform-specific subdirectories to avoid excess CPP. They are not yet used, but will be once we start separating out the larger chunks of platform-specific code.

Context

First few commits of #8901

Priorities and Process

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[edolstra]

This seems to refer to a "unix" subdirectory that doesn't exist.

[Ericson2314]

@edolstra Yes, unix (and windows) directories will be filled in the main PR (#8901) as needed, which actually changes the C++. This is just setting up the infra to do so.

[roberth]

unix (and windows) directories will be filled in the main PR

I agree with Eelco that referring to those in advance is a strange division of labor.
I'd add those references in the commit that actually moves or creates stuff in there.

[Ericson2314]

OK I removed the unix subdirs, just keeping the INCLUDE_* variables that makes it easy to add them.

[Ericson2314]

#10346 follow up issue from review.