Travis speed-ups (#3432)

* Enable ccache
* Cache mingw dependencies
* Expose USE_CCACHE option
* Disable ccache for release builds

.travis.yml

@@ -2,6 +2,10 @@ language: cpp
 compiler: gcc
 dist: trusty
 sudo: required
+cache:
+    directories:
+        - apt_mingw_cache
+        - $HOME/.ccache
 matrix:
     include:
         - env: TARGET_OS=win32
@@ -19,10 +23,14 @@ install:
     - . ${TRAVIS_BUILD_DIR}/.travis/${TRAVIS_OS_NAME}.${TARGET_OS}.install.sh
 before_script:
     - mkdir build && cd build
+    - export CMAKE_FLAGS="-DWANT_QT5=$QT5 -DUSE_WERROR=ON -DCMAKE_BUILD_TYPE=RelWithDebInfo"
+    - if [ -z "$TRAVIS_TAG" ]; then export CMAKE_FLAGS="$CMAKE_FLAGS -DUSE_CCACHE=ON"; fi
 script:
     - . ${TRAVIS_BUILD_DIR}/.travis/${TRAVIS_OS_NAME}.${TARGET_OS}.script.sh
     - make -j4
     - if [[ $TARGET_OS != win* ]]; then make tests && ./tests/tests; fi;
+after_script:
+    - ccache -s
 before_deploy: make package
 deploy:
     provider: releases