Use ccache.

.travis.yml

@@ -9,6 +9,7 @@ matrix:
     - &test-ubuntu
       os: linux
       compiler: gcc
+      cache: ccache
       addons:
         apt:
           packages:
@@ -17,7 +18,7 @@ matrix:
         - COMMON_FLAGS="DYNAMIC_ARCH=1 TARGET=NEHALEM NUM_THREADS=32"
       script:
         - set -e
-        - make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
+        - make $COMMON_FLAGS $BTYPE
         - make -C test $COMMON_FLAGS $BTYPE
         - make -C ctest $COMMON_FLAGS $BTYPE
         - make -C utest $COMMON_FLAGS $BTYPE
@@ -36,6 +37,7 @@ matrix:
 
     - <<: *test-ubuntu
       env:
+        - CC='ccache gcc'
         - TARGET_BOX=LINUX64
         - BTYPE="BINARY=64 USE_OPENMP=1"