* `-lc++` now implies `-lc`.
 * `-lunwind` is now pulled out into a separate `link_libunwind` flag in
   the frontend driver code. This allows a project to request zig to
   provide libunwind even if the default situation that causes it to be
   implicitly added, is not active.
 * build.zig: ask for -lunwind when building the self-hosted compiler on
   Linux. Otherwise we get linker errors with unresolved symbols to
   libunwind.