When building with multiple jobs, the 'Makefile' rule may be executed
before any of its sibling rules, which implicitly create the target
directory. In this case, the attempt to create the symlink for
'Makefile' fails and the 'build_bin_archive' aborts. Analogously to the
sibling rules, this fix creates the target directory as a side effect of
the 'Makefile' rule.
The depot tool did not consistently distinct libraries from regular
targets when handling binary archives. The binary archive of a regular
target is located at <user>/<bin>/<arch>/<name> whereas a library is
located at <user>/<bin>/<arch>/<api>/<name>.
Fixes#2438
This patch replaces the toy downloader (that merely copied files
locally, for testing) with the mechanics needed to download files from a
www server.
It also changes the use of GPG to use detached signatures.
Issue #2339
