libunwind fails to compile with clang-9 as C/C++ compiler

[fogti]

I was using xargo to cross-compile a project (to a musl target) which required the Rust std crate, and it failed while linking building libunwind, because clang-9 doesn't like the combination -std=c99 -std=c++11 used in src/libunwind/build.rs. I was able to work around this by simply commenting out the respective lines, but I think the build.rs script should be able to determine the compiler&version, and skip adding these flags if it detects a recent clang version, which seem to be more strict about this than recent GCC versions (which, as I've read, only produce a warning in that case).

It may help to separate compilation of libunwind's C (-std=c99) and C++ (-std=c++11) code, but I'm unsure how to inform the cc crate that it should use different flags for C and C++, and still linking them into the same static lib...

related:

• Building rustc in Clang 9.0 error: invalid argument '-std=c++11' not allowed with 'C' #69222
• libunwind: also build with C++ compiler on musl #69277

[fogti]

closed as DUP.