[ccache] ccache doesn't handle .i files

[Martin Pool]

If I run ccache 2.2 on a .i file, it thinks it's not a C/C++ file and
it doesn't cache it.

This could be naively fixed by updating check_extension(), but it
would be far better for ccache to know that it doesn't need to run cpp
on such files.  distcc does so.

In particular this means that if ccache is run by distcc or distccd it
will never hit.

This is not a terribly big deal but might be nice to fix.

-- 
Martin