Set API definition if compile language is C++

[Jan200101]

same as #933 but using generator expression which can be used since the project upgraded to cmake 3.10.

This seems to work flawlessly but there are likely some edge cases I did not consider, similar to #933.

Without this PR there are a lot of errors due to extern "C"not being valid in C:

luau-src/VM/include/lua.h:109:1: note: in expansion of macro ‘LUA_API’
  109 | LUA_API lua_State* lua_newstate(lua_Alloc f, void* ud);
      | ^~~~~~~

https://github.com/Jan200101/luau-c-test test repo that prints the _VERSION value in C and C++, the luau repo is commented out in the CMakeLists.txt but can be re-enabled to see the error that would happen without this PR.

[mihaly-sisak]

Tried on my mixed C/C++ project, works flawlessly, gets rid of ugly workarounds.
https://github.com/mihaly-sisak/luau_torch7/blob/master/torch7/torch7.h#L4
Would love to see this merged!

[mihaly-sisak]

Could this please be merged when you get the chance? It’s a really solid contribution and would be great to have in.

Thank you all for the incredible work you’re doing on this project. It’s always impressive to see the level of care and quality that goes into everything.