header only boost install with caching.
header only boost install with caching.

* only redownload if cache folder is not there

* suppress boost install logs

* use brew (and special settings) for mac

* header only boost install

* fix condition checking for cache: apparently cache folder always exist???

* indeed directory exist.

thibault-martinez/iota.lib.cpp,thibault-martinez/iota.lib.cpp,thibault-martinez/iota.lib.cpp