The -Xc99 flag controls compiler recognition of the implemented features from the C99 standard (ISO/IEC 9899:1999, Programming Language -C).
o can be one of the following: all, none.
-Xc99=none disables recognition of C99 features. -Xc99=all enables recognition of supported C99 features.
Specifying -Xc99 without any arguments is the same as -Xc99=all.
The -Xc99 flag cannot be used if the -std or -pedantic flag has been specified.