META_MODE: Follow-up r287865 and define CCACHE_DIR as realpath'd.

Filemon(4) will record paths as they are seen, not as fully resolved. make(1)
will take the .MAKE.META.IGNORE_PATHS values and resolve them. This creates
a discrepancy if CCACHE_DIR is a symlink. Fix this by ensuring it is
resolved for its actual usage.

Submitted by: sjg