overload 'wget' with a function,
so all wget downloads are cached in
ether 'sources' or ENV defined using $CACHEDIR.
directory 'sources' is already in .gitignore
checksum store is in root-dir of repo
and can be manually checked with:
( cd sources && sha256sum -c ../SHA256SUMS.sources )
introducing '$GITDIR' and enforce run in this dir.
Bastian Bittorf