C++ interop via Dynamic Library

[sls1005]

I've discovered an undocumented pragma, `{.exportcpp.}`, that exports a proc 
without `extern "C"`. But I don't think it's a good idea to use it because 
every C++ compiler has its own idea of [name 
mangling](https://en.m.wikipedia.org/wiki/Name_mangling) , which makes your 
library not compatible with other compilers. That's why we need `extern "C"`. 
Besides, the pragma doesn't work for dynlib, but for static lib.