[CFT] Buildworld ccache support

[Simon J. Gerraty]

Bryan Drewery <bdrewery at freebsd.org> wrote:
> https://people.freebsd.org/~bdrewery/patches/world-ccache.diff

In the Junos build - where we used ccache for quite some time
I did:

_CC := ${CC}
CC = ${CCACHE_ENV} ${_CC}

Since sometimes you want the compiler without ccache - eg when linking.

That needs to happen after CC is determined of course.

If the include of bsd.mkopt.mk were moved to after the inlcude of
local.sys.env.mk, then you could simply:

__DEFAULT_NO_OPTIONS+= CCACHE_BUILD

there or in src.sys.env.mk

> 
> To use just set WITH_CCACHE_BUILD= in src.conf or make.conf.  I
> purposely matched it to the same as the ports build.
> 
> Thanks!