o can be one of the following: all, none.
-Xc99=none turns off recognition of C99 features. -Xc99=all turns on recognition of supported C99 features.
Specifying -Xc99 without any arguments is the same as -Xc99=all.
Though the compiler support-level defaults to the features of C99 listed in Table C–6 , the standard headers provided by Solaris software in /usr/include do not yet conform with the 1999 ISO/IEC C standard. If you encounter error messages, try using -Xc99=none to obtain the 1990 ISO/IEC C standard behavior for these headers.