The ARA_SDK contains unguarded is_lock_free calls. This change
allows us to enable ARA compilation on Linux.

This issue made hosts not recognize the ARA capabilities of AU plugins.

This also fixes an issue where the requested font style could be
incorrectly overwritten.

Some hosts on Apple Silicon require that the entire bundle has a
well-formed signature, rather than just the plugin binary itself. This
patch checks whether the bundle signature is valid, and if it is not,
ad-hoc signs the bundle before copying.

With this patch applied, the DemoRunner should build under MinGW, and be
(nearly) feature-complete compared to the MSVC build.

Specifically, when building with MinGW:
- Adds support for accessibility
- Fixes build issues in the juce_video module
- Fixes a link issue in the VST3 wrapper when VST3_CAN_REPLACE_VST2 is
  defined
- Adds support for the new-style native FileChooser
- Tidies up some other low-severity warnings

Known issues:
- Direct2D rendering is still not supported when building with MinGW due
  to ABI compatibilities.