Resolve SDL2, freetype, Vulkan-Headers, SPIRV-Headers, glslang and
SPIRV-Tools from the system instead of the bundled submodules when the
option is ON. It is OFF by default, so the bundled build is unchanged.
Every package is required: a missing one, or Vulkan-Headers older than
1.4.310 (the agc driver uses VK_KHR_maintenance8), fails the configure
with an error and nothing falls back to the submodule. The system
packages are exposed under the same target names the bundled path uses,
so the rest of the CMake is untouched.
SDL2 is linked from the system as a shared library because the prx
libraries are shared objects and distribution static SDL2 is usually not
PIC. ffmpeg-core, libjpeg-turbo, stb, VulkanMemoryAllocator and LibAtrac9
stay bundled.
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