meson: add llvm_extra_link_args for building against a system LLVM

[cyyever]

Problem

src/airconv/meson.build's hardcoded llvm_deps list assumes LLVM was built with -DLLVM_ENABLE_ZSTD=Off (as in docs/DEVELOPMENT.md). When building against a distro/Homebrew LLVM that is built with zstd — e.g. brew install llvm@15 on macOS — its static libs reference ZSTD_* symbols, and the link fails:

Undefined symbols for architecture x86_64:
  "_ZSTD_isError", referenced from: ... libLLVMSupport.a
ld: symbol(s) not found for architecture x86_64

This hits both airconv (native) and winemetal.so. llvm-config --system-libs doesn't report zstd for the Homebrew build, so it can't be discovered automatically.

Change

Add an llvm_extra_link_args array option (default empty), appended to llvm_deps. This lets you build against a system LLVM without compiling LLVM from source:

meson setup ... \
  -Dnative_llvm_path=$(brew --prefix llvm@15) \
  -Dllvm_extra_link_args=/opt/homebrew/opt/zstd/lib/libzstd.dylib,-lz

Default empty → no behaviour change for the documented LLVM build.

Testing

Built win64 (PE dlls + x86_64 unixlib) against Homebrew llvm@15 on macOS 26 (Apple M2) with the option set; airconv and winemetal.so link cleanly and DXMT renders a Unity 2022 D3D11 game at feature level 11.1.