[ccache] win32 patches

[Anuj Goyal]

hey Andrew or Martin, I know using ccache for win32 probably isn't a
high priority (or a priority at all) but do you mind if I submit
patches? or if you are coming out with a new release in the next
couple weeks I can wait. I have 4 files that I have modified (and am
in the process of modifying)

ccache.c   (work in progress, will probably need more community input,
i only changed stuff that I knew about)
execute.c  (come up with a way to use CreateProcess, etc)
unify.c (had to 'map' mmap to the win32 calls)
util.c (very minor)

I am hoping other people use ccache for win32 because VS precompiled
headers are very very hard to use and cause lots of confusion.