Use ccache for linux CI build

.travis.yml

@@ -9,6 +9,10 @@ env:
    # The next declaration is the encrypted COVERITY_SCAN_TOKEN, created
    #   via the "travis encrypt" command using the project repo's public key
    - secure: "jybGzAdUbqt9vWR/GEnRd96BgAi/7Zd1+2HK68j/i/8+/1YH2XxLOy4Jv/DUBhBlJIkxs/Xv8dRcUlFOclZDHX1d/9Qnsqd3oUVkD7k1y7cTOWy9TBQaE/v/kZo3LpzA3xPwwthrb0BvqIbOfIELi5fS5s8ba85WFRg3AX70wWE="
+cache:
+  ccache: true
+  directories:
+  - ${HOME}/.ccache
 addons:
   apt:
     sources:
@@ -17,7 +21,7 @@ addons:
       - llvm-toolchain-xenial-7
     packages: [
       # Dev
-      cmake, clang-7, clang-tools-7, gcc-8, g++-8,
+      cmake, clang-7, clang-tools-7, gcc-8, g++-8, ccache,
       # Boost
       libboost-filesystem-dev, libboost-iostreams-dev, libboost-program-options-dev, libboost-system-dev,
       # FFmpeg

CI/before_script.linux.sh

@@ -14,6 +14,10 @@ if [[ -z "${BUILD_OPENMW}" ]]; then export BUILD_OPENMW=ON; fi
 if [[ -z "${BUILD_OPENMW_CS}" ]]; then export BUILD_OPENMW_CS=ON; fi
 
 ${ANALYZE} cmake \
+    -DCMAKE_C_COMPILER="${CC}" \
+    -DCMAKE_CXX_COMPILER="${CXX}" \
+    -DCMAKE_C_COMPILER_LAUNCHER=ccache \
+    -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
     -DBUILD_OPENMW=${BUILD_OPENMW} \
     -DBUILD_OPENCS=${BUILD_OPENMW_CS} \
     -DBUILD_LAUNCHER=${BUILD_OPENMW_CS} \