CMake loads option defaults from the platform file and then usually
proceeds to write these to the cache, so it is not possible to see
if an option was specified by the user, or was the default.
By setting CMAKE_NOT_USING_CONFIG_FLAGS, we regain control over the
options and can then set this to what we think is suitable, provided
that the user hasn't specified something for us.
If you specified options in an environment variable containing a blank
at the end, then this would not compare equal to the other candidate,
which was stripped, causing the first added argument to drop out! This
is usually the flag for C++11 compliance, causing binary incompatibility
