travis: Gate on some minimal support for incremental compilation.

This commit adds a travis job that builds a stage2 compiler in
incremental mode (but with empty incremental compilation cache).
Building incrementally with an empty cache makes sure that the
compiler doesn't crash in dependency tracking during bootstrapping.

.travis.yml

@@ -34,6 +34,7 @@ matrix:
     - env: IMAGE=x86_64-gnu-llvm-3.7 ALLOW_PR=1 RUST_BACKTRACE=1
     - env: IMAGE=x86_64-musl DEPLOY=1
     - env: IMAGE=x86_64-gnu-distcheck
+    - env: IMAGE=x86_64-gnu-incremental
 
     # OSX builders
     - env: >

src/ci/docker/x86_64-gnu-incremental/Dockerfile

@@ -0,0 +1,27 @@
+FROM ubuntu:16.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+  g++ \
+  make \
+  file \
+  curl \
+  ca-certificates \
+  python2.7 \
+  git \
+  cmake \
+  sudo \
+  gdb \
+  xz-utils
+
+ENV SCCACHE_DIGEST=7237e38e029342fa27b7ac25412cb9d52554008b12389727320bd533fd7f05b6a96d55485f305caf95e5c8f5f97c3313e10012ccad3e752aba2518f3522ba783
+RUN curl -L https://api.pub.build.mozilla.org/tooltool/sha512/$SCCACHE_DIGEST | \
+      tar xJf - -C /usr/local/bin --strip-components=1
+
+RUN curl -OL https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64.deb && \
+    dpkg -i dumb-init_*.deb && \
+    rm dumb-init_*.deb
+ENTRYPOINT ["/usr/bin/dumb-init", "--"]
+
+ENV RUST_CONFIGURE_ARGS --build=x86_64-unknown-linux-gnu
+ENV RUSTFLAGS -Zincremental=/tmp/rust-incr-cache
+ENV RUST_CHECK_TARGET check