[cmake] Fix some macOS compilation errors (#4350)

Don't compile GLSL/GLASM backends without OpenGL enabled. These codepaths are unused on non-OpenGL platforms.

Fixes other miscellaneous errors that pop up on Clang 23 as well.

Signed-off-by: Lizzie lizzie@eden-emu.dev
Signed-off-by: crueter crueter@eden-emu.dev

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

-------------------

Co-authored-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4350
Reviewed-by: crueter <crueter@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

src/video_core/renderer_vulkan/vk_present_manager.cpp

@@ -338,7 +338,7 @@ void PresentManager::PresentThread(std::stop_token token) {
             // By exchanging the lock ownership we take the swapchain lock
             // before the queue lock goes out of scope. This way the swapchain
             // lock in WaitPresent is guaranteed to occur after here.
-            std::exchange(lock, std::unique_lock{swapchain_mutex});
+            void(std::exchange(lock, std::unique_lock{swapchain_mutex}));
             CopyToSwapchain(frame);
 
             // Free the frame for reuse

src/video_core/renderer_vulkan/vk_scheduler.cpp

@@ -300,7 +300,7 @@ void Scheduler::WorkerThread(std::stop_token stop_token) {
             // Exchange lock ownership so that we take the execution lock before
             // the queue lock goes out of scope. This allows us to force execution
             // to complete in the next step.
-            std::exchange(lk, std::unique_lock{execution_mutex});
+            void(std::exchange(lk, std::unique_lock{execution_mutex}));
 
             // Perform the work, tracking whether the chunk was a submission
             // before executing.