Cannot build for musl target

Building llvm-sys for the musl target (on Linux) fails with the following error:

❯ cargo build --target x86_64-unknown-linux-musl
   Compiling llvm-sys v160.0.2 (/home/vadorovsky/repos/llvm-sys.rs)
error: could not find native static library `rt`, perhaps an -L flag is missing?

error: could not compile `llvm-sys` due to previous error

That's because for musl targets, we also try to link libraries mentioned by llvm-config --system-libs dynamically. However, Rust doesn't look up for static libraries in /usr/lib or /usr/lib64 by default. It has to be pointed to all directories with static libraries with cargo:rustc-link-search, even the system-wide ones.

So the proper fix would be specify cargo:rustc-link-search=/usr/lib64 or cargo:rustc-link-search=/usr/lib when linking system libs statically.