Cache ccache output (#4611)

This should reuse object files from unchanged sources between builds.

This brings a rebuild of the Firestore CMake build down from 20 min 37 sec,  to 9 min 13 sec:

  * Before: https://travis-ci.org/firebase/firebase-ios-sdk/jobs/635423433
  * After: https://travis-ci.org/firebase/firebase-ios-sdk/jobs/635295223

#no-changelog

Author: wilhuff

.travis.yml

@@ -2,8 +2,11 @@ os: osx
 osx_image: xcode11.3
 language: objective-c
 cache:
-  - bundler
-  - cocoapods
+  bundler: true
+  cocoapods: true
+  directories:
+    # Built-in support for ccache doesn't seem to pick this up
+    - $HOME/.ccache
 
 stages:
   - checks

CMakeLists.txt

@@ -43,6 +43,7 @@ endif()
 # Enable the ccache compilation cache, if available.
 find_program(CCACHE_PROGRAM ccache)
 if(CCACHE_PROGRAM)
+  message(STATUS "Found ccache: ${CCACHE_PROGRAM}")
   set(CMAKE_C_COMPILER_LAUNCHER   "${CCACHE_PROGRAM}")
   set(CMAKE_CXX_COMPILER_LAUNCHER "${CCACHE_PROGRAM}")
 endif()

scripts/install_prereqs.sh

@@ -126,6 +126,7 @@ case "$PROJECT-$PLATFORM-$METHOD" in
   Firestore-*-cmake)
     brew outdated cmake || brew upgrade cmake
     brew outdated go || brew upgrade go # Somehow the build for Abseil requires this.
+    brew install ccache
 
     # Install python packages required to generate proto sources
     pip install six