Without this, building from source caused:
CommandLine Error: Option 'mc-relax-all' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options
This is due to LLVM static libs compiled in multiple times. But without
the LLVM static libs on the linker line, it caused undefined symbol
linker errors.
So our hands are tied. Homebrew users will have to specify
`-DZIG_PREFER_CLANG_CPP_DYLIB`.