[CFT] Buildworld ccache support

Hi,

If you are interested in using ccache in buildworld please consider
using this patch locally and giving me some feedback.  The current
advice for ccache+buildworld (the CC:= trick in make.conf) actually
results in the build thinking you are using a cross-compiler which
results in unintended side-effects.  It also doesn't work with meta mode
as it handles CC and toolchain differently.  This patch intends to cover
all cases.

https://people.freebsd.org/~bdrewery/patches/world-ccache.diff

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!

-- 
Regards,
Bryan Drewery