format-impl-test fails to link using MacPorts Clang

[jwillikers]

On macOS, the format-impl-test fails to link when using the MacPorts Clang compiler. This is due to the "ifdef apple_build_version" check in the format-impl-test.cc file for value_extractor class. This only checks if the Clang compiler is Apple's Clang, but not whether Apple's libc++ is being linked. Simply replacing the macro with TARGET_OS_MAC corrects my issue, but I don't think this is very robust. I recommend using CMake's CheckCXXSourceRuns to reliably detect whether __int128_t and __uint128_t are available.

[vitaut]

apple_build_version should just be replaced with FMT_USE_INT128.

[jwillikers]

Well... isn't that convenient? :) The test runs successfully when I replace apple_build_version with FMT_USE_INT128.

[vitaut]

Fixed in 73a16b8.