nettle is used by newer versions of gnutls, while older versions of gnutls
used libgcrypt. Support both for now as not every distro has nettle yet.
nettle is preferred as it is more efficient to use and much smaller.
When using an HLS encrypted stream, an assertion failure is thrown:
(gst-launch-1.0:31028): GLib-GObject-WARNING **: cannot register
existing type `GstFragment'
(gst-launch-1.0:31028): GLib-CRITICAL **: g_once_init_leave: assertion
`result != 0' failed
Eventually tracked this down to the call gst_fragment_new()
in function gst_hls_demux_decrypt_fragment.
The GstFragment class is defined in ext/hls/gstfragment.c and in
gst-libs/gst/uridownloader/gstfragment.c. Having two class definitions
with the same name causes the assert failure when trying to allocate
GstFragment. Deleting the version from hls and editing the
Makefile.am solves this assert failure.
https://bugzilla.gnome.org/show_bug.cgi?id=704555
