[ccache] CCACHE_PREFIX included in hash

[Brian Poynor]

Is CCACHE_PREFIX intentionally included in the hash?

For my project it means that objects built locally never match objects 
built remotely with CCACHE_PREFIX=distcc, even though they are
identical, and I'd like to avoid duplicate cache entries.

If interested, I have a patch which avoids hashing it. Currently it is
implemented as an alternate environment variable, CCACHE_NHPREFIX.

-Brian