Using and Porting GNU CC
Chapter 5: Extensions to the C Language Family
GNU C provides several language features not found in ANSI standard C.
-pedantic' option directs GNU CC to print a warning message if
any of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
__GNUC__, which is always defined under GNU CC.
These extensions are available in C and in the languages derived from
it, C++ and Objective C. See C++ Extensions, for extensions that apply only to C++.