This adds a WITH_CCACHE_BUILD knob that will handle using ccache properly.
The ccache guide has advised people to modify CC and CXX to point to
/usr/local/libexec/world/cc, but this causes Makefile.inc1 to consider it
an external compiler, confuses bsd.compiler.mk, and has proved in ports to
be a bad idea due to not being an expected compiler. This also ensures
users are using the world wrappers which handle compiler upgrades properly,
vs default ccache which does not.
Avoid these pitfalls by just adding the world wrappers into the PATH, which
will still cause the ccache wrappers to call the normal CC. This will work
fine whether CC is the bootstrapped cc or an external user-set one.
Obviously this is not complete. Adding mkopt knobs for this and documentation
is needed as well. I just want to get it out for discussion and early testing.
It is working for me great and the pattern has worked in ports for several