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= )
)"
