atomic: detect clang separately #4755
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Description
Original code was assuming that Clang is linked to
libatomicfrom GCC, which's not always the case – Clang can use atomic implementation from its owncompiler-rtlibrary [1].For example, such explicit usage of "native" atomic implementation for Clang is present on Gentoo [2] – in this case, it's possible to create completely GCC-free toolchain. Proposed change should allow such systems to fully utilize atomics from compiler-rt.
Also I think it can fix Android bug mentioned in the comment, since according to [3] and [4], they've slated GCC for removal in v18; while support for required
#ifdefis kinda there [5], I guess it'll be safe to removeif !defined(__ANDROID__)altogether.[1] https://clang.llvm.org/docs/Toolchain.html#atomics-library
[2] https://github.com/gentoo/gentoo/blob/master/sys-libs/libcxx/libcxx-12.0.1.ebuild#L134
[3] https://developer.android.com/ndk/downloads/revision_history
[4] https://en.wikipedia.org/wiki/Android_NDK
[5] android/ndk#1029