ccache support for make buildworld/make release

Folks,

I wonder if anyone considered adding ccache support for 
buildworld/release targets. ccache is a tool, which wraps around gcc, 
allowing to cache object files generated by the compiler to greatly 
speed-up compilation of the same code over and over 
(http://ccache.samba.org/).

It acts as a caching pre-processor to C/C++ compilers, using the -E 
compiler switch and a hash to detect when a compilation can be satisfied 
from cache.  This often results in a 5 to 10 times speedup.

It can benefit both individual developers (less burden to test changes 
via full buildworld), and tinderboxes.

Another interesting use is for package building cluster - it should be 
possible to set-up NFS folder shared among all nodes to hold cached 
objects for all ports in the tree to reduce turnaround time by the order 
of magnitude.

-Maxim