gnu: mlt: Rename to mlt-6.

gnu: mlt: Rename to mlt-6.

* gnu/packages/video.scm (mlt): Rename to mlt-6.
[build-system]: Use cmake-build-system.
[arguments]: Remove #:make-flags. Adjust 'override-LDFLAGS phase.
{#:configure-flags}: Remove --enable-gpl3 and --enable-gpl. Set
"GTK2_GDKCONFIG_INCLUDE_DIR" and "GTK2_GLIBCONFIG_INCLUDE_DIR".
(shotcut): Use mlt-6.
* gnu/packages/animation.scm (synfig): Use mlt-6.
* gnu/packages/kde.scm (kdenlive): Use mlt-6.

This change is to add MLT 7 which will be done in the next commit.

Signed-off-by: Leo Famulari <leo@famulari.name>