Add a USE flag to use system-wide LLVM

[omasanori]

Finally rust-lang/rust#4259 is fixed, so we'll be able to use upstream LLVM. 😃

However, we can't use sys-devel/llvm right now because:

• some patches are not in LLVM 3.4 but 3.5, and
• --llvm-root is not well-tested for now. (recently I found a bug, Build with --llvm-root=$DIR fails when Rust is already installed into $DIR rust-lang/rust#13421)

Anyway, we need an issue to track the progress of that.

[cnd]

@omasanori that's cool news

[jauhien]

See rust-lang/rust#14729

[jauhien]

@omasanori, so it is possible that rust will need downstream patched llvm again? Is it possible to know now how probable is it? Because if it's quit probable there's no sense to deal with llvm unbundling now, until we are less or more sure that upstream llvm can be used for a near time.

[omasanori]

Probably we can use upstream LLVM. To make sure whether we could do so, ask core dev or use Git to check diff.

[jauhien]

One more problem with using system-wide LLVM, when it is used, this linking flag is added:

 -L "/usr/lib64" 

And if you have rust already installed you get this:

rustc: x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib/libsync
/home/jauhien/work/rust/rust/src/libsync/lib.rs:37:1: 37:21 error: multiple dylib candidates for `rustrt` found
/home/jauhien/work/rust/rust/src/libsync/lib.rs:37 extern crate rustrt;
                                                   ^~~~~~~~~~~~~~~~~~~~
/home/jauhien/work/rust/rust/src/libsync/lib.rs:37:1: 37:21 note: candidate #1: /home/jauhien/work/rust/rust/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustrt-4e7c5e5c.so
/home/jauhien/work/rust/rust/src/libsync/lib.rs:37 extern crate rustrt;
                                                   ^~~~~~~~~~~~~~~~~~~~
/home/jauhien/work/rust/rust/src/libsync/lib.rs:37:1: 37:21 note: candidate #2: /usr/lib64/librustrt-4e7c5e5c.so
/home/jauhien/work/rust/rust/src/libsync/lib.rs:37 extern crate rustrt;
                                                   ^~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
make: *** [x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib/stamp.sync] Error 101

[omasanori]

@jauhien maybe it is related to rust-lang/rust#13421

[jauhien]

@omasanori, yes, exactly this. I see you pointed it before. Thanks.

[jauhien]

Solved, I will import it to the tree in a week.

[omasanori]

Great work! I'll test it tonight. 👍