__extension__ can be put in front of expressions

__extension__ can be put in front of expressions

glib uses __extension__ in macros dealing with 64 bits integer such as
GUINT64_SWAP_LE_BE().

To quote the GCC manual:
  `-pedantic' and other options cause warnings for many GNU C extensions.  You
  can prevent such warnings within one expression by writing `__extension__'
  before the expression.

https://bugzilla.gnome.org/show_bug.cgi?id=605779