Additions for build.rs by q66, necessary for our musl setup.

From 1eb558f246269606c6d8d73824ef6b44fa10764e Mon Sep 17 00:00:00 2001
From: Samuel Holland <samuel@sholland.org>
Date: Sat, 9 Sep 2017 00:14:16 -0500
Subject: [PATCH 06/16] Prefer libgcc_eh over libunwind for musl

---
 src/libunwind/lib.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libunwind/lib.rs b/src/libunwind/lib.rs
index 9182e349b19..0377fbb58fc 100644
--- a/library/unwind/src/lib.rs
+++ b/library/unwind/src/lib.rs
@@ -51,7 +51,7 @@
         #[link(name = "unwind", cfg(not(target_feature = "crt-static")))]
         extern "C" {}
     } else {
-        #[link(name = "unwind", kind = "static", modifiers = "-bundle", cfg(target_feature = "crt-static"))]
+        #[link(name = "gcc_eh", cfg(target_feature = "crt-static"))]
         #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
         extern "C" {}
     }