The CMAKE_NOT_USING_CONFIG_FLAGS option is really only settable by a
user that doesn't want _DEBUG or _RELEASE flags to not have effect.
If we want other flags than the platform default, we must do hairy
things like clearing them from the cache (overriding the user's
seletion if it was the same as the platform default).
The operator precedence in CMake can be surprising, so add parenthesis
to make in unambigious. Also, make it more explicit from which language
we get the default options. (They are all set from the settings given
for C++; why would have different optimization options for C and C++?)
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
Roland Kaufmann