Drop some compatibility code.

Apparently, Meson propagates the linker flags but not the compiler
flags from a `static_library`'s dependencies list.

Closes #392

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!