Make LadspaUGen optional

[gusano]

Fixes #259

[patrickdupuis]

I get this error when trying to run cmake -DLADSPA=OFF ..

CMake Error at source/CMakeLists.txt:363 (install):
  install TARGETS given target "ladspalist" which does not exist in this
  directory.

[gusano]

@patrickdupuis doh.. I fixed it and force pushed.. sorry for that and thanks for spotting

[patrickdupuis]

Thanks! Tested on Linux.

[mossheim]

There should be more documentation than this -- maybe a known issues section in the readme?

[gusano]

@brianlheim note added in the readme

[totalgee]

This frequent source of confusion ("why does scsynth fail to boot on my Linux distro?") came up again in the TidalCycles forums recently:
https://club.tidalcycles.org/t/supercollider-wont-boot/2285

The "solution" of deleting the LADSPA plugin locally was discussed, and I sent a link to this PR. The comment was made that hopefully it would be merged soon... ;-)

[totalgee]

Thanks @brianlheim !