meson: Use feature options

meson: Use feature options

By using meson features we can replace the handcrafted dependency
auto-detection by just passing the value from get_option directly to the
required arg for dependency, find_library etc.
'auto' features make the dependency optional, 'enabled' requires it
while 'disabled' features will skip detection entirely.

Any skipped or not found dependency will just be a no-op when passed to
build steps and therefore we can also skip the creation of empty vars.

The use of skip_deps for these is dropped here as meson provides a way
to disable all optional features in one go by passing
'-Dauto_features=disabled'.