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

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.

This change should mean that if JUCE is installed using one compiler
(e.g. clang) and then consumed in a build that uses a different compiler
(e.g. gcc), the helper targets will use the correct flags for both
compilers.