make libsamplerate optional
--- a/avidemux_plugins/cmake/admCheckAudioDeviceLibs.cmake
+++ b/avidemux_plugins/cmake/admCheckAudioDeviceLibs.cmake
@@ -116,7 +116,7 @@ IF(JACK_FOUND)
 	FIND_HEADER_AND_LIB(SRC samplerate.h samplerate src_get_version)
 	PRINT_LIBRARY_INFO("Secret Rabbit Code" SRC_FOUND "${SRC_INCLUDE_DIR}" "${SRC_LIBRARY_DIR}")
 
-	IF(SRC_FOUND)
+	IF(SRC AND SRC_FOUND)
 		SET(USE_SRC 1)
 	ENDIF()