Add 'CONFIG+=disable-prebuilts' qmake option

When specified, qmake uses pkg-config for libraries on macOS instead of the prebuilts in the libs submodule.

app/app.pro

@@ -52,7 +52,7 @@ win32 {
     # Work around a conflict with math.h inclusion between SDL and Qt 6
     DEFINES += _USE_MATH_DEFINES
 }
-macx {
+macx:!disable-prebuilts {
     INCLUDEPATH += $$PWD/../libs/mac/include
     INCLUDEPATH += $$PWD/../libs/mac/Frameworks/SDL2.framework/Versions/A/Headers
     INCLUDEPATH += $$PWD/../libs/mac/Frameworks/SDL2_ttf.framework/Versions/A/Headers
@@ -64,7 +64,7 @@ macx {
     QMAKE_OBJECTIVE_CFLAGS += -F$$PWD/../libs/mac/Frameworks
 }
 
-unix:!macx {
+unix:if(!macx|disable-prebuilts) {
     CONFIG += link_pkgconfig
     PKGCONFIG += openssl sdl2 SDL2_ttf opus
 
@@ -152,13 +152,17 @@ win32:!winrt {
     CONFIG += soundio discord-rpc
 }
 macx {
-    LIBS += -lssl -lcrypto -lavcodec.61 -lavutil.59 -lopus -framework SDL2 -framework SDL2_ttf
+    !disable-prebuilts {
+        LIBS += -lssl -lcrypto -lavcodec.61 -lavutil.59 -lopus -framework SDL2 -framework SDL2_ttf
+        CONFIG += discord-rpc
+    }
+
     LIBS += -lobjc -framework VideoToolbox -framework AVFoundation -framework CoreVideo -framework CoreGraphics -framework CoreMedia -framework AppKit -framework Metal -framework QuartzCore
 
     # For libsoundio
     LIBS += -framework CoreAudio -framework AudioUnit
 
-    CONFIG += ffmpeg soundio discord-rpc
+    CONFIG += ffmpeg soundio
 }
 
 SOURCES += \