Fix Clang Detection

[Lord-Kamina]

For several versions now, CMake by default refers to macOS’ Clang as AppleClang instead of just Clang, which would fail STREQUAL. Fixed by changing it to MATCHES.

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[Lord-Kamina]

CLA signed.

[googlebot]

CLAs look good, thanks!

[coveralls]

Coverage remained the same at 89.256% when pulling 6b764fe on Lord-Kamina:master into a8082de on google:master.

[AppVeyorBot]

✅ Build benchmark 1506 completed (commit b2f8488aae by @Lord-Kamina)

[dmah42]

Thanks!