schannel: use Win8 CERT_NAME_SEARCH_ALL_NAMES_FLAG with old SDKs

[vszakats]

Define CERT_NAME_SEARCH_ALL_NAMES_FLAG macro if missing.
To allow using a runtime branch regardless of build-time SDK version,
when running on Windows 8+.

In practice it enables this branch for builds using mingw-w64 v3, and
MSVC with Windows SDK <8.

Also reducing build variations.

Follow-up to 29e40a6 #4761 #3711
Follow-up to 8996300 #1325
Follow-up to 172b2be #264 (comment)

[Copilot]

Pull request overview

This PR enables the use of Windows 8+ certificate name search functionality at runtime regardless of the SDK version used at build time. This is achieved by defining the CERT_NAME_SEARCH_ALL_NAMES_FLAG macro when it's not provided by the SDK, allowing older build environments (mingw-w64 v3 and MSVC with Windows SDK <10) to take advantage of this feature when running on Windows 8 or later.

Key changes:

• Moves the CERT_NAME_SEARCH_ALL_NAMES_FLAG check inside the runtime Windows 8 compatibility check
• Defines the flag locally when the SDK doesn't provide it, enabling runtime branching
• Removes unnecessary (void) casts that were previously needed to suppress unused variable warnings

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[vszakats]

Thanks @jay!