In Meson 0.60.0, a new custom dependency was added, called 'iconv'. It's dependency check can be found at https://github.com/mesonbuild/meson/blob/b1d2282078127df0afed522e97b6c498ff5d80f9/mesonbuild/dependencies/misc.py#L380
There may or may not also be a bunch of other custom dependencies which can be found at https://github.com/mesonbuild/meson/blob/b1d2282078127df0afed522e97b6c498ff5d80f9/mesonbuild/dependencies/__init__.py#L199
it would be nice to have some doc, or a current example, to help adding these custom dependencies
also, more doc here : https://mesonbuild.com/Dependencies.html