Set the correct dependencies in the pkg-config file

When linking statically, the including project needs to know what the
current library build depends on so they can link to it. Store this
information in the pkg-config file.

While here, remove claims that users need to link to zlib or libcrypto.
README: add a note about the optional dependencies
transport: let the progress output return an error

There are any number of issues that can come up in the progress
callback, and we should let the user cancel at that point as well.
Merge pull request #1906 from libgit2/cmn/net-cb-errors

transport: let the progress output return an error
Merge pull request #1905 from libgit2/cmn/list-deps

Be explicit about dependencies