Warning for duplicate define of MRB_ARY_NO_EMBED

[lopopolo]

mruby disables inline arrays for 32-bit architectures:

mruby/include/mruby/array.h

Lines 24 to 30 in 7ecc16f

	#if defined(MRB_32BIT) && defined(MRB_NO_BOXING) && !defined(MRB_USE_FLOAT32)
	# define MRB_ARY_NO_EMBED
	# define MRB_ARY_EMBED_LEN_MAX 0
	#else
	# define MRB_ARY_EMBED_LEN_MAX ((mrb_int)(sizeof(void*)*3/sizeof(mrb_value)))
	mrb_static_assert(MRB_ARY_EMBED_LEN_MAX > 0, "MRB_ARY_EMBED_LEN_MAX > 0");
	#endif

I've been -DMRB_ARY_NO_EMBED in my own builds of mruby.

This set of #defines does not check whether MRB_ARY_NO_EMBED is already defined, which leads to a build warning on clang:

warning: /Users/lopopolo/dev/artichoke/playground/target/wasm32-unknown-emscripten/release/build/artichoke-backend-61ddbc16c386408b/out/artichoke-mruby/mruby/include/mruby/array.h:25:10: warning: 'MRB_ARY_NO_EMBED' macro redefined [-Wmacro-redefined]
warning: # define MRB_ARY_NO_EMBED
warning:          ^
warning: <command line>:12:9: note: previous definition is here
warning: #define MRB_ARY_NO_EMBED 1