Add a Makefile setting for buildign with ccache.

This greatly speeds up the build when frequently rebuilding from scratch
(e.g. in case of a buildbot). Some hacks are added for using ccache in
combination with Clang.

Some numbers (on high-end consumer hardware):
```
Build type: distcleanall -> make julia-debug
Including all deps (without download), built using Clang, LLVM+Debug+Assertions

Initial ccache build:
real    12m51.734s
user    81m21.656s
sys     5m34.256s

Subsequent full rebuilds:
real    3m59.535s
user    18m12.236s
sys     1m28.120s
```