CCACHE_BUILD: Only export CCACHE_PATH= if it was already set with a value.
Older ccache don't work with an empty CCACHE_PATH value. They will error with:
ccache: FATAL: Could not find compiler "cc" in PATH make: "/mnt/bdrewery/git/onefs/src/share/mk/bsd.compiler.mk" line 134: Unable to determine compiler type for /usr/local/bin/ccache cc. Consider setting COMPILER_TYPE.
Sponsored by: EMC / Isilon Storage Division