Use ccache on AppVeyor to speedup builds

[vchuravy]

This might speedup our AV builds a bit.

[vchuravy]

This looks good on AV. We will have to see in subsequent builds how much that speeds up the build.

Generally speaking we are right now really close to the 2h mark, which is the cutoff time on AV.
We currently don't cache the BinaryBuilder LLVM download but that should happen in the background while we build other projects.

Current stats are:

cache directory                     /cygdrive/c/ccache
cache hit (direct)                     4
cache hit (preprocessed)              33
cache miss                           450
called for link                       13
called for preprocessing              10
compile failed                         1
preprocessor error                     2
bad compiler arguments                 1
unsupported source language           24
autoconf compile/link                 13
no input file                        132
files in cache                      1067
cache size                         113.2 Mbytes
max cache size                       1.0 Gbytes

[vtjnash]

We currently don't cache the BinaryBuilder LLVM download

Aren’t we serving these files off S3 already (the same as the cache files)? If we are seeing any issues, we should just make sure we’re replicated into the right AWS region.