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
Merge pull request #1905 from libgit2/cmn/list-deps

Be explicit about dependencies