manual SDL2 library search

[jpcima]

This detects SDL2 without using the FindModule shipped with the library,
which is unreliable. It's the manual search which works in all configurations.

("old way" copied back from libOPNMIDI)
see #196 (comment)

[jpcima]

I rewrote the whole PR @Flamefire thanks to your advice, do you think it's fine?

[Flamefire]

Problem now is that you duplicate the conditions used in the subfolders. They might easily become out-of-sync. I'd suggest either:

• Duplicate the target check at the point of usage. This is kinda fine as one sees why things are done
• Create a very simple find module containing basically your code for ADLMIDI_SDL2 and find_package that.
• Create a wrapper function like function(find_sdl2_targets) find_package(SDL2 ${ARGV}) <your code> and include+call that: find_sdl2_targets(REQUIRED) target_link_libraries(... SDL2::SDL2)

[jpcima]

Ok to merge this @Wohlstand ?
It's an annoyance to work from the master branch, because of the cmake failure.
(the case when SDL2 was built with cmake, if I understand right)

[Wohlstand]

Looks much better! Yeah, I think, the same set of works are must go into libOPNMIDI's build too