travis: Use ccache to speed up builds

.travis.yml

@@ -4,6 +4,8 @@ os:
 
 language: c
 
+cache: ccache
+
 env:
     global:
         secure: "YKG77M7zMvJ+IeV2ziw//HcHqMqFoAzIZlE99Yw/aOn5pvMYKq6Ep7EFVhbfDu9yN0T7M5csCGQeH7/ylDlsZSRMUw72844ezMDM8e10T/lW/T/OYN7j1ZVHh3WSJgS+1D9VG6/Y0OY1Si3lb7PcOdAIU0fPJV5xQONN2+hpJeI="
@@ -18,6 +20,7 @@ before_install:
     - if [ $TRAVIS_OS_NAME = osx ]; then brew install cpanm bison flex berkeley-db lmdb openldap openssl; fi
     - if [ $TRAVIS_OS_NAME = osx ]; then sudo cpanm install JSON; fi
     - if [ $TRAVIS_OS_NAME = osx -a -n "$COVERAGE" ]; then brew install jq; fi
+    - if [ $TRAVIS_OS_NAME = osx -a -n "$COVERAGE" ]; then brew install ccache; export PATH="/usr/local/opt/ccache/libexec:$PATH"; fi
     - mkdir ci-build
     - mkdir coverity-build
     - ./autogen.sh