Add ccache support to build (via --enable-ccache configure option)

[pnkfelix]

The current docs suggest that one should enable ccache by playing games with what gcc (and clang, etc) map to in one's PATH.

That is certainly one approach to the problem.

But I would prefer if it were not the only way to get the effect; I don't want to globally reroute my system's gcc/clang to ccache, and would prefer a configure option that makes our build Makefiles use the form of invoking ccache that passes along compiler as well.

I've hacked up a patch to do this on my system, so I'll assign this to myself.

[pnkfelix]

(Note that even though the Makefile infrastructure will go away with #2237, that is orthogonal to whether we easily allow ccache to be substituted in.)

[Aatch]

Closed by #6813

[pnkfelix]

Hmm, my patch is not as effective as I had hoped.

My current hypothesis is that this is because our build process is including absolute paths in the preprocessor output (i.e. becauase the -Idir directives are using absolute paths for the dirs), and by default ccache will treat such files as different keys in its cache.

One way to fix this would be to make use of the CCACHE_BASEDIR environment variable. Another could be to use relative paths instead of absolute ones at appropriate steps in the build...