Travis/CCache: workaround a ccache bug with g++-4.8.1

From ccache's manual:

    If compilation doesn’t work with ccache, but it works without it, one
    possible reason is that the compiler can’t compile preprocessed output
    correctly. A workaround that may work is to set CCACHE_CPP2.

Signed-off-by: David Wagner <[email protected]>

Author: dawagner

.travis.yml

@@ -52,6 +52,8 @@ before_script:
     - if [ "$CC" = "gcc" ]; then
           export CC=gcc-4.8 CXX=g++-4.8;
           coverage=ON;
+          # Workaround a bug with the ccache/g++-4.8.1 combination
+          export CCACHE_CPP2
       fi
 
 # how to build