Update to 3.1.8:

ccache 3.1.8
------------
Release date: 2012-08-11

New features and improvements
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    - Made paths to dependency files relative in order to increase cache hits.
    - Added work-around to make ccache work with buggy GCC 4.1 when creating a
      pre-compiled header.
    - Clang plugins are now hashed to catch plugin upgrades.

Bug fixes
~~~~~~~~~

    - Fixed crash when the current working directory has been removed.
    - Fixed crash when stderr is closed.
    - Corrected a corner case when parsing backslash escapes in string
      literals.
    - Paths are now correctly canonicalized when computing paths relative to
      the base directory.

Other
~~~~~

    - Made git version macro work when compiling outside of the source
      directory.
    - Fixed `static_assert` macro definition clash with GCC 4.7.

(wiz)