[mlt-git] Build using cmake.

[bartoszek]

As of https://github.com/mltframework/mlt/commit/b520e0a775ab3ea532fdabe846c530f97e254ba5 MLT drops Autotools build system in favor of CMake.

Please fill in any missing CMake flags, this is only a rudimental sketch setting just release type and install prefix.

Also, I've allowed myself to refactor your pkgver() in accordance with ArchLinux AUR wiki guidelines 😏