Re-running CMake causes Android builds to recompile everything when building Qt itself

Every time CMake is re-run, CMAKE_CXX_FLAGS_DEBUG and CMAKE_C_FLAGS_DEBUG get another copy of -fno-limit-debug-info added. CMAKE_CXX_FLAGS_RELEASE and CMAKE_C_FLAGS_RELEASE get another copy of -O2 -DNDEBUG added. These flags are coming from the Android NDK, but the fault is actually in qtbase where in QtFlagHandlingHelpers.cmake we write these variables back into the cache. We should never do that, it isn't safe because of exactly the sort of scenario happening here.