Add libjack.so.0 fallback logic for AppImages (#3958)

Add libjack.so.0 fallback logic

src/core/CMakeLists.txt

@@ -1,5 +1,14 @@
 IF(LMMS_HAVE_WEAKJACK)
 	set(WEAKJACK core/audio/AudioWeakJack.c)
+
+	# Build libjack.so.0 stub as weakjack.so for AppImages
+	IF(LMMS_BUILD_LINUX)
+		ADD_LIBRARY(weakjack MODULE ../../src/core/audio/AudioWeakJack.c)
+		INCLUDE_DIRECTORIES("${CMAKE_SOURCE_DIR}/include")
+		# We can't predict an AppImage build, so stash the build artifact for later
+		INSTALL(TARGETS weakjack LIBRARY DESTINATION "${CMAKE_BINARY_DIR}/optional")
+		SET_TARGET_PROPERTIES(weakjack PROPERTIES PREFIX "" SUFFIX ".so")
+	ENDIF()
 ENDIF()
 	
 set(LMMS_SRCS