macOS: There's no -D switch in BSD install

[ajelinski]

One of the latest commits (c613a35) added the install -D command.

After that change building the plugins on macOS fails in the https://github.com/litex-hub/litex-conda-eda repository.

The BSD's install -d seems to have a similar meaning to the GNU's install -D but I'm not sure about that:

     -d      Create directories.  Missing parent directories are created as required.

It'd be great if the script could either have install used universally to both implementations or simply have the commands replaced in the commit with install as a fail-over for macOS.

[hzeller]

Oh, sorry, just seen this bug assignment. Will make something that should work in both.

[timkpaine]

I'm working on this FYI. I have 100% working Mac build locally, just trying to deal with propagating changes through all the deps. My changes remove all the conda stuff (separately I'm moving surelog / UHDM onto conda-forge, I'd like to obviate the need for a separate litex-hub channel ideally).

[timkpaine]

@hzeller I'm not a member of this repo I don't think but feel free to assign off to me