.travis.yml: Improve ccache use

Per https://ccache.dev/manual/latest.html:

  By default, ccache tries to give as few false cache hits as
  possible. However, in certain situations it’s possible that
  you know things that ccache can’t take for granted.

  [The CCACHE_SLOPINESS environment variable] makes it possible
  to tell ccache to relax some checks in order to increase the
  hit rate.

We can relax the ctime/mtime header checks:

  - include_file_ctime

    By default, ccache also will not cache a file if it
    includes a header whose ctime is too new. This option
    disables that check.

  - include_file_mtime

    By default, ccache will not cache a file if it includes
    a header whose mtime is too new. This option disables
    that check.

We also add a call to clear the cache statistics before running
the build, and display them when the build finishes.

See https://docs.travis-ci.com/user/caching/#ccache-cache

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -70,6 +70,9 @@ env:
     - TEST_CMD="make check -j3 V=1"
     # This is broadly a list of "mainline" softmmu targets which have support across the major distros
     - MAIN_SOFTMMU_TARGETS="aarch64-softmmu,arm-softmmu,i386-softmmu,mips-softmmu,mips64-softmmu,ppc64-softmmu,riscv64-softmmu,s390x-softmmu,x86_64-softmmu"
+    - CCACHE_SLOPPINESS="include_file_ctime,include_file_mtime"
+    - CCACHE_MAXSIZE=1G
+
 
 git:
   # we want to do this ourselves
@@ -77,10 +80,13 @@ git:
 
 
 before_script:
+  - command -v ccache && ccache --zero-stats
   - mkdir -p ${BUILD_DIR} && cd ${BUILD_DIR}
   - ${SRC_DIR}/configure ${BASE_CONFIG} ${CONFIG} || { cat config.log && exit 1; }
 script:
   - make -j3 && travis_retry ${TEST_CMD}
+after_script:
+  - command -v ccache && ccache --show-stats
 
 
 matrix: