Enable ccache for macOS CI

CI/before_install.osx.sh

@@ -6,6 +6,7 @@ brew install https://gist.githubusercontent.com/nikolaykasyanov/f36da224bdef4202
 brew switch cmake 3.12.4
 brew outdated pkgconfig || brew upgrade pkgconfig
 brew install qt
+brew install ccache
 
 curl -fSL -R -J https://downloads.openmw.org/osx/dependencies/openmw-deps-110f3d3.zip -o ~/openmw-deps.zip
 unzip -o ~/openmw-deps.zip -d /private/tmp/openmw-deps > /dev/null

CI/before_script.osx.sh

@@ -10,6 +10,8 @@ cd build
 
 cmake \
 -D CMAKE_PREFIX_PATH="$DEPENDENCIES_ROOT;$QT_PATH" \
+-D CMAKE_C_COMPILER_LAUNCHER=/usr/local/opt/ccache/bin/ccache \
+-D CMAKE_CXX_COMPILER_LAUNCHER=/usr/local/opt/ccache/bin/ccache \
 -D CMAKE_OSX_DEPLOYMENT_TARGET="10.9" \
 -D CMAKE_OSX_SYSROOT="macosx10.14" \
 -D CMAKE_BUILD_TYPE=Release \