package/gstreamer1/gstreamer1: convert package type to meson

package/gstreamer1/gstreamer1: convert package type to meson

Gstreamer 1.16.0 is the last version to support autotools, and will be removed
in the next version.

Other changes:
  - The meson build provides no 'disable-valgrind' option but checks
    for the availability of valgrind/valgrind.h, so add an optional
    valgrind dependency instead.

  - Add the following new meson-specific options:
  - -Dgtk_doc=disabled
  - -Dglib-asserts=disabled
  - -Dglib-checks=disabled
  - -Dgobject-cast-checks=disabled

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>