[ccache] Win32 support.

[Joel Rosdahl]

Regarding the "Win32 support" patch:

I suspect that the static inline functions in ccache.h will result in
portibility problems, so I think it's better to just move them to util.c.

Maybe the definition and use of is_win32_dir should be surrounded by
#ifdef _WIN32 guards?

Why is a check for both "ccache" and "ccache.exe" needed? Doesn't
argv[0] on Windows always end with the actual extension? The reason I'm
asking is that if only "ccache.exe" is needed, we could maybe use the
EXEEXT variable from autoconf and avoid the conditional compilation.

Why is the special handling of shell scripts needed?

We should probably use PATH_DELIM for CCACHE_EXTRAFILES as well.

Otherwise it looks good from my perspective.

-- Joel