configure: move target-specific defaults to an external machine file

Enable Windows-specific defaults with a machine file, so that related
options can be automatically parsed and included in the hel

configure: move target-specific defaults to an external machine file

Enable Windows-specific defaults with a machine file, so that related
options can be automatically parsed and included in the help message.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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