[travis] Cache protobuf compilation.

.travis.yml

@@ -3,6 +3,12 @@ dist: trusty
 sudo: required
 language: cpp
 
+cache:
+  apt: true
+  directories:
+    - $BUILD_CCACHE_DIR
+    - $HOME/.ccache
+    - $HOME/.cache/pb
 addons:
   apt:
     sources:
@@ -24,6 +30,7 @@ jobs:
     - env:
         - TEST_NAME=ASAN
       install:
+        - export PATH="/usr/lib/ccache:$PATH"
         - mkdir build && cd build
         - CC=clang-6.0 CXX=clang++-6.0 cmake -G Ninja
           -DCMAKE_BUILD_TYPE=Release -DGLOW_WITH_OPENCL=OFF -DGLOW_WITH_CPU=ON

utils/install_protobuf.sh

@@ -1,5 +1,9 @@
 #!/bin/sh
 set -ex
+
+pb_dir="$HOME/.cache/pb"
+mkdir -p "$pb_dir"
+
 wget https://github.com/google/protobuf/releases/download/v2.6.1/protobuf-2.6.1.tar.gz
-tar -xzf protobuf-2.6.1.tar.gz
-cd protobuf-2.6.1 && ./configure CC=gcc CXX=g++ CXX_FOR_BUILD=g++ --prefix=/usr && make && sudo make install
+tar -xzf protobuf-2.6.1.tar.gz -C "$pb_dir" --strip-components 1
+cd "$pb_dir" && ./configure CC=gcc CXX=g++ CXX_FOR_BUILD=g++ --prefix=/usr && make -j2 && sudo make install