avoid using uninitialised OPENFLIPPER_PRODUCT_STRING in make

added warning and fallback, if OPENFLIPPER_PRODUCT_STRING is not set

cmake/OpenFlipper_common.cmake

@@ -14,7 +14,12 @@ macro (acg_modify_project_dirs)
       file (MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/Build/${ACG_PROJECT_BINDIR})
     endif ()
   elseif (APPLE)
-    set (MAC_OPENFLIPPER_BASEDIR "${OPENFLIPPER_PRODUCT_STRING}.app/Contents/Resources")
+    if(OPENFLIPPER_PRODUCT_STRING)
+      set (MAC_OPENFLIPPER_BASEDIR "${OPENFLIPPER_PRODUCT_STRING}.app/Contents/Resources")
+    else()
+      message(WARNING "OPENFLIPPER_PRODUCT_STRING is not set, using OpenFlipper as fallback")      
+      set (MAC_OPENFLIPPER_BASEDIR "OpenFlipper.app/Contents/Resources")
+    endif()
     set (ACG_PROJECT_DATADIR "${MAC_OPENFLIPPER_BASEDIR}")
     set (ACG_PROJECT_PLUGINDIR "${MAC_OPENFLIPPER_BASEDIR}/Plugins")
     set (ACG_PROJECT_LIBDIR "${MAC_OPENFLIPPER_BASEDIR}/../MacOS")