missing LILV_DEFAULT_LV2_PATH definition after switch to meson build

The waf build system used to define LILV_DEFAULT_LV2_PATH depending on the platform, see https://gitlab.com/lv2/lilv/-/blob/c36d3451278555134de850fc548d743b6da2179a/wscript#L252

It looks like these bits were lost after switching to the meson build system.

I've tried the following change lilv-lv2_path.patch and it works here (Gentoo, libdir set to lib64)