meson: fix ladspa dependencies

meson: fix ladspa dependencies

There are two issues, both related to dependency checking with the meson
support for the ladspa plugin.

With autotools, lrdf is handled like an optional dependency. But with
meson it is required. This makes the meson support less flexible and
inconsistent with autotools.

When autotools is used it properly checks if ladspa.h is available.
But with meson it does not, instead it treats lrdf as the main
dependency. This could cause a build failure if lrdf is installed, but
the ladspa sdk is not.

https://bugzilla.gnome.org/show_bug.cgi?id=794350