The following macro makes it possible to use features of Posix that are extensions to C, as well as platform extensions not defined by Posix.
This macro was introduced in Autoconf 2.60. If possible, enable extensions to C or Posix on hosts that normally disable the extensions, typically due to standards-conformance namespace issues. This should be called before any macros that run the C compiler. The following preprocessor macros are defined where appropriate:
Enable extensions on GNU/Linux.
Enable general extensions on Solaris.
Enable threading extensions on Solaris.
Enable extensions for the HP NonStop platform.
Enable extensions for AIX 3, and for Interix.
Enable Posix functions for Minix.
Enable additional Posix functions for Minix.
Identify Minix platform. This particular preprocessor macro is obsolescent, and may be removed in a future release of Autoconf.