Add x86_64-unknown-linux-musl build support

Author: martell

.travis.yml

@@ -76,6 +76,10 @@ matrix:
       env: TARGET=s390x-unknown-linux-gnu
            DOCKER=s390x-unknown-linux-gnu
            SKIP_TESTS=1
+    - os: linux
+      env: TARGET=x86_64-unknown-linux-musl
+           DOCKER=x86_64-unknown-linux-musl
+           SKIP_TESTS=1
 
     # Android use a local docker image
     - os: linux

README.md

@@ -610,6 +610,7 @@ platform of your choice:
 - [x86_64-pc-windows-msvc](https://static.rust-lang.org/rustup/dist/x86_64-pc-windows-msvc/rustup-init.exe)<sup>[†](#vs2015)</sup>
 - [x86_64-unknown-freebsd](https://static.rust-lang.org/rustup/dist/x86_64-unknown-freebsd/rustup-init)
 - [x86_64-unknown-linux-gnu](https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-gnu/rustup-init)
+- [x86_64-unknown-linux-musl](https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-musl/rustup-init)
 - [x86_64-unknown-netbsd](https://static.rust-lang.org/rustup/dist/x86_64-unknown-netbsd/rustup-init)
 
 <a name="vs2015">†</a>

ci/docker/x86_64-unknown-linux-musl/Dockerfile

@@ -0,0 +1,11 @@
+FROM alpine:3.8
+
+RUN apk update && \
+  apk add \
+  curl \
+  ca-certificates \
+  perl \
+  make \
+  gcc
+
+ENV CC_x86_64_unknown_linux_musl=gcc