[Bug 755989] configure: Add --enable-libssh2 switch and make it independent of Curl

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=755989

--- Comment #4 from Carlos Rafael Giani <dv at pseudoterminal.org> ---
True. The reasoning behind this is that curl *may* use libssh2, but libssh2
does not depend on curl and is not related to it in any way. The nested libssh2
check in curl is in my opinion a too strong coupling. It just a bit cleaner to
factor out libssh2.

And, the reason why I added a configure switch for libssh2 was that in Yocto,
you want to make sure that optional features are switched on/off explicitely
through an OpenEmbedded feature called "packageconfig". The way the libssh2
check currently works is that it enables ssh2 support in Curl if libssh2 is
present. 

This can lead to indeterministic cross compilation builds. Example:
Given the same build flags and the same build machine, a gst-plugins-bad build
on setup A produces a different result than setup B - because something else
just happened to build libssh2 earlier.

For this reason, in Yocto, by default, dependency-less GStreamer plugins are
enabled, and the others disabled, unless their dependencies are part of
openembedded-core. See this for example:
http://git.openembedded.org/openembedded-core/tree/meta/recipes-multimedia/gstreamer/gstreamer1.0-plugins-bad.inc

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