schannel: use Win8 `CERT_NAME_SEARCH_ALL_NAMES_FLAG` with old SDKs

schannel: use Win8 `CERT_NAME_SEARCH_ALL_NAMES_FLAG` with old SDKs

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 29e40a6d8a70630dd8eaa15beded205792342d08 #4761 #3711
Follow-up to 899630021153b2a26a43008cccc6620b6c3f9bbf #1325
Follow-up to 172b2beba6b89b632c09be7a88645e3a0607cfe9 #264 (comment)

Closes #20000