[osx/ios] - enable gnutls

This enables gnutls for ios and osx. This means all platforms have gnutls support (so instead of excluding it - we have gnutls enabled by default with this PR for all platforms).

The issues we had might still be there on osx but only show on osx 10.6 which we will drop soon (because of C++11 compatibility for example). So lets enable it and see what users report.

The iOS was never enabled because the dependencies didn't compile - i fixed it now (the asm code of gmp will not compile with the ios assambler - so i had to disable it - i also found that this is the "official" way of compiling gmp for ios).

I tested it on osx and ios with the test https streams from the forum thread here:

http://forum.kodi.tv/showthread.php?tid=209569

@wsnipex for a pair of eyes.