This change gives projects more flexibility to e.g. pre-build
juce_audio_plugin_client into a staticlib and to reuse it in several
targets.

The CoreAudioKit (and on macOS, AudioUnit) frameworks are required to
host AudioUnit plugins. Hosts (especially those which don't use the
`juce_audio_utils` module) should use the new `PLUGINHOST_AU` parameter
to `juce_add_*` in order to add the correct preprocessor definition and
link the necessary frameworks.

If git/zip-extraction add hidden directories containing files with a
`.h` extension we don't want to try to build those files as PIPs. Using
GLOB instead of GLOB_RECURSE should help to avoid files in nested
directories.

Fixes an issue where VST3 plugins would appear to link successfully, but
fail to load in hosts due to missing symbols.