Switch to downloading prebuilt release artifacts instead of a submodule

app/app.pro

@@ -36,6 +36,10 @@ DEFINES += QT_DEPRECATED_WARNINGS
 DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0
 
 win32 {
+    !exists($$PWD/../libs/windows) {
+        error("Missing dependencies. Please run 'powershell .\setup-deps.ps1' to fetch prebuilt libraries.")
+    }
+
     contains(QT_ARCH, x86_64) {
         LIBS += -L$$PWD/../libs/windows/lib/x64
         INCLUDEPATH += $$PWD/../libs/windows/include/x64 $$PWD/../libs/windows/include/x64/SDL2
@@ -49,6 +53,10 @@ win32 {
     LIBS += ws2_32.lib winmm.lib dxva2.lib ole32.lib gdi32.lib user32.lib d3d9.lib dwmapi.lib dbghelp.lib
 }
 macx:!disable-prebuilts {
+    !exists($$PWD/../libs/mac) {
+        error("Missing dependencies. Please run 'python3 setup-deps.py' to fetch prebuilt libraries.")
+    }
+
     INCLUDEPATH += $$PWD/../libs/mac/include $$PWD/../libs/mac/include/SDL2
     LIBS += -L$$PWD/../libs/mac/lib
 }