getentropy function as been added to MacOS 10.12 and iOS 10.
During GnuTLS compilation, the function can be detected in the new
10.12 SDK, but at runtime it fails to find the appropriate method on
the system.

It means that we can produce packages that won't work on some OS
versions. For precaution this commit patches GnuTLS to stop trying to
enable this function on all macOS versions.

It also removes another patch in GnuTLS for OSX that wasn't used anymore

Change-Id: I3af8b111f65e38d2d01da600c899d5717e82d8e9
Tuleap: #911