#286 - fix hang when >1 Camomile LV2 plugin is loaded on Linux

[hyperpenelope]

This issue was ultimately caused by an object buried in the JUCE code having an inappropriate scope - GCC made the object a "unique global symbol", which means there is only one instance across the current process. When multiple plugins were loaded, they all shared this instance, but did not share other relevant objects (from the JUCE code), causing havoc. See #286 for al the gory details.

The fix is to ban the compiler from marking symbols as "unique global".

Also set the C++ standard to be C++20 for the Camomile_LV2 meta-plugin, in line with the other meta-plugins.