Makefile: use ccache when available

[pjonsson]

Use ccache if it is found in PATH
and the CCACHE variable is not
already defined (=user hasn't called
make as "make CCACHE='' ..").

On some quick tests with building
the container, I get a 50% hitrate
with a warm cache. When there is
a cache hit, ccache will copy
the file from the cache instead
of compiling it, and that is much
quicker.

[pjonsson]

I have used ccache for the past decade, and I have never had any problems with it, it just works and does exactly what it is meant to do without any fuss. It's also a well-established software by now, there's support for it in CMake and probably a bunch of other build systems.

Despite making such a glowing recommendation for ccache, I would still avoid using it when building release binaries, but that is just to be certain that release binaries are pristine.