Thirdparty benchmark: Fix Clang detection.

[Lord-Kamina]

CMake 3.0+ refers to macOS’ Clang as AppleClang, which would fail a STREQUAL check. Fixed by changing it to MATCHES.

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[nlohmann]

The code we use is some release of Google Benchmarks (https://github.com/google/benchmark/releases). Your fix should (also) be added there, and maybe it is already fixed there in a new release.

[coveralls]

Coverage remained the same at 100.0% when pulling e842706 on performous:fix-clang-detection into b911654 on nlohmann:develop.

[Lord-Kamina]

A quick look at master says it hasn't been fixed upstream. I guess I'll submit it over there also.

EDIT: Submitted upstream already also. google/benchmark#697

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!