COMP: Provide ITK_MSVC_STATIC_CRT to in-tree build

[dg0yt]

ITK_MSVC_STATIC_CRT was exported from installed ITKConfig.cmake, but
it wasn't available when building external modules (e.g. RTK) together
with ITK. This caused linking errors for tools. This PR is the minimal change to ensure consistent configuration.

ITK_MSVC_STATIC_CRT is used in CMake/ITKModuleExternal.cmake and in CMake/UseITK.cmake. The external RTK module includes the use file at
https://github.com/RTKConsortium/RTK/blob/d15c02af5d36098b5209e846c0585e70290fe6b7/applications/CMakeLists.txt#L32-L33
and started to use the wrong CRT from this point due to ITK_MSVC_STATIC_CRT being not set.

This issue was noticed in CI for the vcpkg port. The previous workaround was passing -DITK_MSVC_STATIC_CRT=ON to CMake. I don't expect this to be widely used.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

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[dzenanz]

I would think that this change would have no effect.

[SimonRit]

Looks good to me, thanks!

[thewtex]

@dg0yt thank you!