Fix #6805: add --enable-ccache configure option

Fix #6805: add --enable-ccache configure option to prefix compiler invocations with `ccache` to attempt to reuse common results, e.g. for LLVM (re)builds.

The information at developer [Note-ccache](../../wiki/Note-ccache) and at [ccache and clang concerns](http://petereisentraut.blogspot.fr/2011/09/ccache-and-clang-part-2.html) were what drove my introduction of the `-Qunused-arguments` and `CCACHE_CPP2` options.  (Though I did confirm first-hand that at least the first really is necessary.)

Yes, one certainly can re-route how `gcc` and `clang` are resolved in one's PATH and use that as a way to invoke `ccache`.  But I personally do not want to introduce that change to my own PATH, and this seems like a small enough change that it does not hurt to add it, at least for now.  (I don't know what form it would take when we move over to `rustpkg`.)