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

share/mk/local.meta.sys.mk

@@ -193,7 +193,9 @@ UPDATE_DEPENDFILE= NO
 .MAKE.META.BAILIWICK = ${SB} ${OBJROOT} ${STAGE_ROOT}
 
 .if defined(CCACHE_DIR)
+CCACHE_DIR := ${CCACHE_DIR:tA}
 .MAKE.META.IGNORE_PATHS += ${CCACHE_DIR}
+.export CCACHE_DIR
 .endif
 
 CSU_DIR.${MACHINE_ARCH} ?= csu/${MACHINE_ARCH}