Install ccache on OSX and add GTSAM_BUILD_WITH_CCACHE flag to cmake

.travis.sh

@@ -23,6 +23,7 @@ function build_and_test ()
   fi
 
   cmake $SOURCE_DIR \
+      -DGTSAM_BUILD_WITH_CCACHE=ON
       -DGTSAM_BUILD_UNSTABLE=$GTSAM_BUILD_UNSTABLE \
       -DGTSAM_BUILD_EXAMPLES_ALWAYS=$GTSAM_BUILD_EXAMPLES_ALWAYS \
       -DGTSAM_BUILD_TESTS=$GTSAM_BUILD_TESTS

.travis.yml

@@ -19,6 +19,10 @@ addons:
 before_install:
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update          ; fi
 
+install:
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew install ccache  ; fi
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then export PATH="/usr/local/opt/ccache/libexec:$PATH" ; fi
+
 script:
   - bash .travis.sh