[ccache] Some Modification about CCACHE_BASEDIR

[xiaoxiao]

Hi:
After anaylsing the code of ccache, we find that if we set the
environment variable of CCACHE_BASEDIR, ccache will translate the paths
to the relative ones. However, if multiple users are developing in one
machine, maintain the value of the variable could be a burden for them.
Thus, I did some modification to the code, in order to get the base dir
dynamically, I add another enviroment variable. Once this variable is
set, the ccache would fetch the working directory, the value of PWD, as
the base dir. After this modification, the problem I mentioned above
seems to be settled.
A problem arose here, could this modification bring any potential
problem? If not, may I make a push request on any branch of your code.

Thanks~

Sean