mruby-bigint doesn't compile in c++ project

Hello there,

in commit [61aa223](https://github.com/mruby/mruby/commit/61aa2234d8304db79ba1fd63b3e2e72946ed362a) 
this error gets thrown when using bigint in a c++ mgem:
```
mruby/mrbgems/mruby-bigint/core/bigint.c:12:
mruby/include/mruby/throw.h:26:3: error: #error "need to be compiled with C++ compiler"
   26 | # error "need to be compiled with C++ compiler"
```