Enable ccache for build

.travis.yml

@@ -1,5 +1,6 @@
 ---
 language: generic
+cache: ccache
 addons:
   apt:
     sources:
@@ -24,7 +25,7 @@ env:
 install:
   - pip install --user cpp-coveralls
 script:
-  - ./numake tests WITH_LTO=0 WITH_UBSAN=1 WITH_ASAN=1
+  - ./with-ccache ./numake tests WITH_LTO=0 WITH_UBSAN=1 WITH_ASAN=1
 after_success:
   - if [ "$COMPILER" = "gcc" ]; then coveralls --root . --build-root . --exclude .build/*/src/tests --exclude .build/*/src/lip/vendor --exclude .build/*/src/lip/khash_impl.c ; fi
 sudo: required

with-ccache

@@ -0,0 +1,5 @@
+#!/bin/sh -e
+
+export CC="ccache ${CC:-cc}"
+export CXX="ccache ${CXX:-c++}"
+exec $*