Cache CMake distribution between builds

.travis.yml

@@ -1,6 +1,10 @@
 sudo: false
 dist: bionic
 language: c++
+cache: 
+  apt: true
+  directories:
+    - $HOME/.deps
 
 matrix:
   include:
@@ -68,15 +72,19 @@ matrix:
             - valgrind
 
 install:
-  - cd "$TRAVIS_BUILD_DIR"
+  - mkdir -p $HOME/.deps
   - |
     if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
-      CMAKE_URL="https://cmake.org/files/v3.16/cmake-3.16.3-Linux-x86_64.tar.gz"
-      mkdir -p cmake && travis_retry wget --no-clobber --no-check-certificate --quiet -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake
-      export PATH=${TRAVIS_BUILD_DIR}/cmake/bin:${PATH}
+      if [[ ! -d "$HOME/.deps/cmake" ]]; then
+        cd $HOME/.deps
+        CMAKE_URL="https://cmake.org/files/v3.16/cmake-3.16.3-Linux-x86_64.tar.gz"
+        mkdir -p cmake && travis_retry wget --no-clobber --no-check-certificate --quiet -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake
+      fi
+      export PATH=${HOME}/.deps/cmake/bin:${PATH}
       echo ${PATH}
     fi
   - cmake --version
+  - cd "$TRAVIS_BUILD_DIR"
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then cmake -DNO_THIRDPARTY_CONTAINERS=ON -G Xcode .; fi
   - if [[ "$TRAVIS_OS_NAME" != "osx" ]]; then cmake -DBUILD_LUA_BINDINGS=ON -DBUILD_SAMPLES=ON -DDISABLE_RTTI_AND_EXCEPTIONS=${DISABLE_RTTI_AND_EXCEPTIONS:-OFF} -DNO_THIRDPARTY_CONTAINERS=${NO_THIRDPARTY_CONTAINERS:-OFF} -DNO_FONT_INTERFACE_DEFAULT=${NO_FONT_INTERFACE_DEFAULT:-OFF} -G Ninja .; fi