intl: Pacify pre-C99 compilers

intl: Pacify pre-C99 compilers

* gettext-runtime/intl/plural-exp.h (HAVE_STRUCT_INITIALIZER): New
macro, based on the check previously in plural-exp.c.  Also add check
for __SUNPRO_C for Solaris Studio C compiler.
(GERMANIC_PLURAL): Define as 'struct expression' not 'const struct
expression' if struct/union initializers are not supported by the
compiler.
* gettext-runtime/intl/plural-exp.c: Use HAVE_STRUCT_INITIALIZER.
Reported by Dagobert Michelsen and investigated by pan7 in:
https://savannah.gnu.org/support/?108743