use system library on openbsd

[semarie]

compiler_rt is provided by default on OpenBSD, so use it instead of
rebuilding it from source.

[alexcrichton]

Hm I don't think this is correct? Surely OpenBSD doesn't have __rust_probestack

[semarie]

The purpose is to avoid recompiling compiler_rt (what build.rs does), and make the crate uses the system library instead of.

I used this way to avoid a missing symbol (from emutls) while compiling rustc (since 1.23.0, I think). It was related to librustc_binaryen if I correctly remember. On OpenBSD the symbol is only available via libcompiler_rt, but I couldn't mix the system library with the compiled one from compiler-builtins (where emutls isn't compiled).

Without such change, I have the following error while compiling rustc (nightly):

rustc:/data/semarie/build-rust/build_dir/build/x86_64-unknown-openbsd/stage1/lib/rustlib/x86_64-unknown-openbsd/codegen-backends/librustc_trans-llvm.so: undefined symbol '__emutls_get_address'
error: couldn't load codegen backend "/data/semarie/build-rust/build_dir/build/x86_64unknown-openbsd/stage1/lib/rustlib/x86_64-unknown-openbsd/codegen-backends/librustc_trans-llvm.so": "Cannot load specified object"

Avoiding to compiling the C part at all and using libcompiler_rt.a from system solve the issue.

If I didn't mess myself __rust_probestack is in the rust part. So it will be compiled and linked with the system library.

[alexcrichton]

Ah right yes sorry, got mixed up!