This patch introduces a new CMake macro add_lldb_plugin_subdirectory to be used by the plugins. It's similar to add_lldb_tool_subdirectory which makes it possible to disable certain tools at configuration time. With the new macro, the same is possible for plugins. The difference is that it includes the plugin's parent directory in the name as well, so that the variable is named LDB_PLUGIN_SYMBOLFILE_PDF_BUILD rather than LDB_PLUGIN_PDB_BUILD.
I've updated the macOS CMake cache to give an idea of what this looks like.