Add ccache support for Linux on Travis

.travis/linux/build.sh

@@ -1,3 +1,3 @@
 #!/bin/bash -ex
 
-docker run -v $(pwd):/yuzu ubuntu:18.04 /bin/bash /yuzu/.travis/linux/docker.sh
+docker run -e CCACHE_DIR=/ccache -v $HOME/.ccache:/ccache -v $(pwd):/yuzu ubuntu:18.04 /bin/bash /yuzu/.travis/linux/docker.sh

.travis/linux/docker.sh

@@ -1,12 +1,18 @@
 #!/bin/bash -ex
 
 apt-get update
-apt-get install --no-install-recommends -y build-essential git libqt5opengl5-dev libsdl2-dev libssl-dev python qtbase5-dev wget cmake ninja-build
+apt-get install --no-install-recommends -y build-essential git libqt5opengl5-dev libsdl2-dev libssl-dev python qtbase5-dev wget cmake ninja-build ccache
 
 cd /yuzu
 
+export PATH=/usr/lib/ccache:$PATH
+ln -sf /usr/bin/ccache /usr/lib/ccache/cc
+ln -sf /usr/bin/ccache /usr/lib/ccache/c++
 mkdir build && cd build
+ccache --show-stats > ccache_before
 cmake .. -DYUZU_BUILD_UNICORN=ON -DCMAKE_BUILD_TYPE=Release -G Ninja
 ninja
+ccache --show-stats > ccache_after
+diff -U100 ccache_before ccache_after || true
 
 ctest -VV -C Release