mruby-bigint: avoid C99 compound literal in MPZ_CTX_INIT

MPZ_CTX_INIT used a compound literal with designated initializers:

  mpz_ctx_t ctx##_struct = ((mpz_ctx_t){.mrb = ..., .pool = ...});

Both features are C99-only, and are not accepted by legacy C++
compilers (notably gcc 4.x) when mruby is pulled into a C++
translation unit via the -cxx.cxx wrapper.

Replace with plain member assignment so the macro expands to code
that is valid under C89/C++98 as well.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bigint/core/bigint.c

@@ -50,11 +50,16 @@ typedef struct mpz_context {
   mpz_pool_t *pool;  /* NULL for heap-only operations */
 } mpz_ctx_t;
 
-/* Convenience macros for context creation */
+/* Convenience macros for context creation.
+ * Uses per-member assignment instead of a C99 compound literal with
+ * designated initializers so the file compiles as C++ on legacy
+ * toolchains (pre-C++20). */
 #define MPZ_CTX_INIT(mrb_ptr, ctx, pool_ptr) \
   mpz_pool_t pool ## _storage = {{0}};\
   mpz_pool_t *pool_ptr = &pool ## _storage;\
-  mpz_ctx_t ctx ## _struct = ((mpz_ctx_t){.mrb = (mrb_ptr), .pool = (pool_ptr)}); \
+  mpz_ctx_t ctx ## _struct; \
+  ctx ## _struct.mrb = (mrb_ptr); \
+  ctx ## _struct.pool = (pool_ptr); \
   mpz_ctx_t *ctx = &(ctx ## _struct);
 
 /* Access macros for readability */