Fix build error for MRB_WORD_BOXING

debug record size assertion fail

Merge branch 'modify_toolchains_gcc' of https://github.com/crimsonwoods/mruby into crimsonwoods-modify_toolchains_gcc

get_filename_table_size realloc fix.

Updated the documentation to better match the way things work now that 172cd281 is merged.

'CXX' is used instead of 'CC'.
'CXXFLAGS' is used instead of 'CFLAGS'.

Newly added compile option '-std=gnu99' is not necessary for C++.
And 'g++' should be used to compile for C++ source codes, I think.

Automatically detect toolchain if it is visualcpp, and hence we drop vs2010.rake altogether - it’s no longer needed.

build with MRB_GC_FIXED_ARENA on travis

Automatically detect toolchain if it is visualcpp, and hence we drop vs2010.rake altogether - it’s no longer needed.

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)