From the GCC 4.6 Release notes‡, 4.6 already supports: Static assertions Typedef...

From the GCC 4.6 Release notes‡,

4.6 already supports:

  Static assertions
  Typedef redefinition
  New macros in <float.h>
  Anonymous structures and unions

4.7 supports:

  Unicode strings
  the predefined macros __STDC_UTF_16__ and __STDC_UTF_32__
  Nonreturning functions 
  Alignment support
  A built-in function __builtin_complex is provided to support C library implementation of the CMPLX family of macros.

I haven't found any good sources for what is to be included in 4.8

‡ http://gcc.gnu.org/gcc-4.6/changes.html#c ‡ http://gcc.gnu.org/gcc-4.7/changes.html