intl: Assume ISO C 99 or newer.
* gettext-runtime/intl/plural-exp.h (HAVE_STRUCT_INITIALIZER): Remove macro.
(GERMANIC_PLURAL): Assume HAVE_STRUCT_INITIALIZER is 1.
* gettext-runtime/intl/plural-exp.c: Assume HAVE_STRUCT_INITIALIZER is 1.
(init_germanic_plural): Remove function.
(INIT_GERMANIC_PLURAL): Remove macro.
(EXTRACT_PLURAL_EXPRESSION): Update.
