You will not see "arena overflow" error anymore, but I encourage gem authors
to check your gems with MRB_GC_FIXED_ARENA to avoid memory broat.

declare struct mrb_irep.

Fixed so that it builds with VS2013. VLA:s are not supported, because of its strict C89 world view.

renamed mrb_exc_new3 to mrb_exc_new_str

codegen_malloc/realloc: static + simple mem alloc

GCC: add -std=gnu99 to CFLAGS

- this change prevents issues like #1539 in the future
- Clang's default is already gnu99 but GCC's is gnu89
- mruby is C99 (+ GNU exts), not C89 (+ GNU exts)

- more descriptive name (there's no mrb_exc_new2)
- no wrapper (macro/static inline function) for
backwards compatibility because nothing besides
mruby itself seems to be calling this function:
<http://mruby-code-search.ongaeshi.me/home?query=mrb_exc_new3>

- there's no reason for both to have external linkage
- use simple versions of memory allocation functions
which are returning NULL instead of longjmp-ing,
so codegen_error will be called to cleanup.

irep->pool[n].type should be IREP_TT_*, not MRB_TT_*

fix warning: "implicit conversion from enumeration type 'enum mrb_vtype' to different enumeration type 'enum irep_pool_type' [-Wenum-conversion]"

fix length check in read_section_debug()

Changed a variable declaration to top of block