Merge pull request #301 from afbjorklund/depend_mode

Add depend mode (avoid the preprocessor).

When using run_second_cpp (which is the default now), ccache executes
the preprocessor just to determine the object hash, and will do so for
every cache miss. While compiling, the same work is done again.

When executing a massively parallel build using ccache and distcc,
the system controlling the build can get a fairly high load because of
all these ccache-only preprocessor executions.

This is based on the work of Geert Kloosterman with remaining items addressed.

Closes #234.