multimedia/mlt7: Backport bugfixes and some style changes to Makefile

Backport following upstream commits:
52c5bde61947fb473413cd9daf4b2664769702d9
f7adc156a7aa5081cde9f52ca52d67df8bb728cf
1d3669e60cd416e42ddeea171a9e1116ed1b613b
2266e5c8e06efb18f3f13f26f0e7b6147ba5a840
2df4fc8e630dbab5e9f1fa1fbf69b859ec8c159d
b4943c55ea44b6d72a5f22b5dcb754c3c89454e3
8a821cc5864c848bb55c251a87fdf0dae66e6c81

References:
https://gitlab.archlinux.org/archlinux/packaging/packages/mlt/-/commit/945df5433b8bfb3024699bf54c61ca6b2a9f06aa
https://github.com/mltframework/mlt/commit/52c5bde61947fb473413cd9daf4b2664769702d9
https://github.com/mltframework/mlt/commit/f7adc156a7aa5081cde9f52ca52d67df8bb728cf
https://github.com/mltframework/mlt/commit/1d3669e60cd416e42ddeea171a9e1116ed1b613b
https://github.com/mltframework/mlt/commit/2266e5c8e06efb18f3f13f26f0e7b6147ba5a840
https://github.com/mltframework/mlt/commit/2df4fc8e630dbab5e9f1fa1fbf69b859ec8c159d
https://github.com/mltframework/mlt/commit/b4943c55ea44b6d72a5f22b5dcb754c3c89454e3
https://github.com/mltframework/mlt/commit/8a821cc5864c848bb55c251a87fdf0dae66e6c81

PR: 273792
Approved by: kde (tcberner)
Sponsored by: Blinkinblox