Selecting_a_Standard

   You can define a feature-test macro to select each standard. You
   can do this either with a #define preprocessor directive in your
   C source before the inclusion of any header file, or with the
   /DEFINE qualifier on the CC command line.

   Features not defined by one of the previously named standards are
   considered VSI C extensions and are selected by not defining
   any standards-related, feature-test macros.

   If you do not explicitly define feature test macros to control
   header file definitions, you implicitly include all defined
   symbols as well as VSI C extensions.
 

Additional information available:

_XOPEN_SOURCE_EXTENDED          _XOPEN_SOURCE   _POSIX_C_SOURCE==199506
_POSIX_C_SOURCE==2              _POSIX_C_SOURCE==1
_STDC_VERSION__==199409_        _ANSI_C_SOURCE
Interactions_with_the_/STANDARD_Qualifier