llvm-libunwind - Use sys-libs/llvm-libunwind instead of sys-libs/libunwind This would be similar to graphicsmagick and libressl USE flags.
sys-libs/libunwind and sys-libs/llvm-libunwind are ABI-incompatible, at least due to different sonames of libraries. Switching between them requires rebuilding of all reverse dependencies. Example usage: RDEPEND="libunwind? ( llvm-libunwind? ( sys-libs/llvm-libunwind:0= ) !llvm-libunwind? ( sys-libs/libunwind:0= ) )"