Consider using ccache

[jmlapre]

ccache is a tool that caches (the hash of) files and compiler flags such that subsequent identical compilations can be returned much faster. The Trilinos project uses it and has gotten decent speedups. We should consider using it as well.

[jwilso]

If you show me how to configure it, we can coordinate on getting this working.

[jmlapre]

So to get started, install ccache and set the following environment variables:

export CCACHE_NODISABLE=true
export CCACHE_DIR=${CCACHE_BIN/ccache}
export CCACHE_BASEDIR="$HOME" # we may not need this setting
export CCACHE_NOHARDLINK=true
export CCACHE_UMASK=002
export CCACHE_MAXSIZE=100G

We need to make sure that we have a large enough volume to hold all of the objects that get generated when we do our nightly tests. 100G is what we use on Trilinos; we may not need that much space. After that it's a lot of tinkering for the most part to check if the cache is behaving as expected...