libv4l: ensure libv4l gets built, fixes mediastreamer build

libv4l: ensure libv4l gets built, fixes mediastreamer build

In commit f837251785e9389f53d695ddb8a094707865171b ("package/libv4l:
allow build of v4l2 utilities on noMMU platforms"), Hugues Fruchet
added a bunch of patches to libv4l in order to allow the v4l utilities
to be built on noMMU platforms.

However, as part of those patches, he entirely disabled the build of
libv4l in static linking configurations, because libv4l uses
dlopen(). Unfortunately, this breaks the build of applications like
mediastreamer in static linking configurations, and generally makes
our libv4l packages a little bit awkward: you can enable it, but it
doesn't install anything (in static linking configurations).

A closer look shows that dlopen() is only used by libv4l for plugin
support, and libv4l only provides one single plugin, and its build is
already conditional. Therefore, this commit adds yet another patch to
libv4l, which re-enables the build of libv4l, but disables the
plugin-related logic when plugin support is disabled (and it was
already automatically disabled in static linking configurations).

While at it, we update the comment in libv4l.mk that lists the patches
that makes autoreconf necessary.

Fixes the build of mediastreamer:

  http://autobuild.buildroot.net/results/af091cfd0508df9395778cdc796f77e95c168410/

Cc: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>