This allows WITH_CCACHE_BUILD to be set and still allow modifying MAKE_ENV/CC,
etc, to achieve using the ccache wrappers located at LOCALBASE/libexec/ccache.
This kind of mechanism is needed to support ccache-memcached which has
many build dependencies. Building a version of that first then allows
building further ports with it without introducing cyclic dependencies.
Poudriere will grow support for using a static version of ccache for this
purpose.
With hat: portmgr
in the global scope as bsd.ccache.mk expected.
This was causing ccache to use WKRDIR/.ccache for obj caching. 'make clean'
would then lose all cached files. This worked in in Poudriere though as
CCACHE_DIR is explicitly defined.
With hat: portmgr
