Mk/bsd.ccache.mk: Add CCACHE_ENABLED variable for ports' convenience

Some projects do some special handling when the ccache program is present.
This causes problems because ports framework has its own ccache settings
and mere presence of ccache program doesn't mean that ccache is enabled.
The new CCACHE_ENABLED variable can be used to tell projects if ccache
is really enabled.

PR: 266310
Approved by: tcberner@ (as portmgr)